As designed Registration: User can create an account with the same username as a previously deleted user

[Harpers Tate]

....if an employee that is working is suddenly no longer working and we would like to remove his account but would like to keep everything they did what do we do ? In Wordpress when you delete a user you are presented the option to assign these contents to another user for example.

Obviously, without knowing all the subtle nuances of your own site, this suggestion may mean little. However, FWIW:
- if your primary need is to preserve the old account, its name, EMail, contents and so so in an effectively frozen state, then banning it does all these things.
- if your primary need is to reassign that account's "work" to another account, then you can do a merge (i.e. without resorting to SQL or similar). This process deletes the old account (thus, releasing its name and Email, etc., for re-use) and theoretically transfers all associated material to the new. It's a somewhat flawed process in fact (which is why I said "theoretically") as some add-ons don't quite behave, and the core XF search doesn't return content which WAS from the old account. Note that things like "join date" on the new account also get updated to the old data.
I guess anything that amounts to a mixture of these would need something coding for you.