Is that a difference between exchanges and webwallets?
I googled on the exchange kraken
kraken.com closing account
and here seems the be a ticket form: "closing account"
Nothing related
It depends on the company itself and its policy, a web wallet or an exchange doesn't matter. For example, you can close your Coinbase account (I did it) but can't close your blockchain.com account. You can do it with Kraken.com but can't do it with Bitstamp.
So it's not like: you can close an account on an exchange but can't with a web wallet. As I said it totally depends on the enterprise (for security reasons, cleaning the database, laws,...)
Change the password and the email and you're done