A lot depends on the software, the main reason that deleting accounts can be tricky, is as Peter mentioned, often when an account is deleted or closed, there's not always options, and that member will also have his/her posts removed too. So this can, and often does ruin many threads.
The same applies when a member putting photos into their post, if they delete the photos, the photos vanish from the threads, again ruining that thread. So, some forums host the photos themselves to stop this happening but, this becomes expensive for the forum owners, as it eats up bandwidth.
There is some forum software, that allows the deleting or banning of a member, and option boxes can be checked, that delete or save that users posts. I have admin'd forums in the past, some had this option some didn't. The ones that didn't meant going into the data base which is a bit of a chore. Some members want to leave without trace, including any posts or photos they made, this obviously can really mess things up, especially on an informative thread, where a lot of discussion and photos were used to explain. If one persons input, and possibly photos are gone, that thread will make very little sense.
A better way for everybody, is if the member concerned changes their email to one they don't use, remove any personal info from their profile, then ask a moderator to change their name, and then just stop posting.
This is very quick and easy to do.