Clients leave. How that goes determines whether they speak well of you afterwards, whether you keep the ones who hear about it, and whether you are still holding data you should not be.
It is also the point where a relationship that ended amicably can turn sour over a handover that took too long.
Hand over the data promptly
Their site, their databases, their email. Provide a full account backup they can take elsewhere, or transfer it directly to their new provider if they ask.
Do this quickly and without making it difficult. A slow handover is remembered as obstruction whether or not it was, and the goodwill costs you nothing to keep.
It is also worth being clear about what a backup contains and what it does not, DNS records, cron jobs and PHP settings are configuration rather than data, and a client discovering those are missing blames you in place of the format, what to check before moving hosting is the client-side version worth sending them.
Do not hold data hostage over a dispute
If there is an unpaid invoice, pursue it as a debt. Withholding a client's own data is a different thing, and in many jurisdictions it is not a position you want to defend.
Separate the two: hand over the data, chase the invoice. The alternative turns a billing disagreement into a complaint about you, publicly, at exactly the moment the client is talking to their peers about providers.
Keep the account until the move completes
Do not terminate on the day they ask. Suspend if you must, but keep the data.
Migrations run late, mail keeps arriving on your server while DNS caches expire, and the client will discover something missing three days in. Keeping the account for a week or two costs you almost nothing and prevents the one conversation that ends badly.
Agree a date in writing: "your account will be removed on the 15th, please confirm your migration is complete." That makes the deletion a scheduled event in place of a surprise.
The domain, if you hold it
If the domain is registered in your account or under your reseller, the client needs it released.
Unlock it, provide the authorisation code, and do not delay. A domain is the client's identity, not part of your service, and a transfer takes days on its own without you adding to them.
If the domain is genuinely yours (you registered it as part of a package they never paid separately for) that is a conversation to have explicitly and early, not something to leave them to discover at the registrar. Getting an EPP code explains what they will ask for.
Then actually delete it
Once the agreed date passes and the move is confirmed, remove the account.
Holding a former client's site indefinitely is not helpfulness; it is data you are responsible for, on a server you are responsible for, with no reason to have it. It counts against your disk, it is in your backups, and if it is ever exposed it is your problem.
Take one final archive, keep it for a defined period if your terms say so, and delete the rest.
Clean up what the account left behind
Terminating an account does not always remove everything.
Check for DNS zones still on your servers, which will keep answering for a domain you no longer host and can send visitors to a site that is not there: particularly on a cluster, where a zone can persist on a member you did not check. Maintaining a DNS cluster goes into removing zones properly.
Then check for mail routing entries, leftover subdomains under your own domains, and any monitoring still watching a site that is gone.
Cancel the billing properly
Stop the recurring charge at your end rather than waiting for it to fail.
An invoice generated after a client left produces a support conversation, a refund, and a bad impression: all avoidable by closing the subscription when you close the account. Integrating billing and automation explains wiring the two together.
Ask why, once
A single short question, with no attempt to change their mind: what prompted the move?
The answers cluster. Price, one incident handled badly, a feature you do not offer, or a developer who prefers another provider. Each suggests something different, and you only learn it if you ask at the moment they are willing to say.
Do not make it a survey and do not argue with the answer. You are collecting information, not running a retention campaign.
Leave the door open
Some clients come back, usually within a year, and usually because the alternative turned out to be worse in a way they did not anticipate.
A clean, prompt, unresentful exit is what makes that possible. A last message saying the account is closed, the data was handed over, and they are welcome back costs one minute.
It also matters for the ones who stay. Clients talk to each other, and how you handle someone leaving is visible in a way your uptime is not. There is more on the rest of what is worth writing down before you need it in reseller best practices.
Write the process down
Handover format, how long you keep the account, when you delete, what happens to the domain, how billing stops.
Put it in your terms so clients agree to it at the start rather than negotiating it at the end. Every part of this is easier when both sides already know what happens.
When the ending is a disputed charge instead of a notice period, the records you kept decide it. How to Handle a Chargeback or Billing Dispute goes into which ones.
The largest version of a client leaving is all of them moving to somebody else on purpose. For arranging that well, see How to Value and Sell a Hosting Business.