Technically we could remove all posts of a banned member but anybody who quoted them would still have those quotes in the messages, which would just create a mess in the discussions that included posts by a banned member.
If you were previously banned, reinstatement is always an eventual possibility which is why we leave the posts and the account on the site.
I'd say 9 out of 10 times somebody asks us to delete an account they are doing so in haste and after a few days reconsider their request. If we were to delete the account and thousands of posts, there would be no way to restore it without shutting the entire site down, merging from a backup, etc, which would literally cost us about $1,000 in fees since we pay by the hour for that type of database work.
Granted, if somebody is really sure that they want to leave and never ever return and are okay with their account being lost, we could change their username to guest and reassign all posts, thereby removing their name/username/signature from the site. But we'd have the rights to keep the content on the site.
Whenever we get a request to remove old content that could be harmful or damaging to a member, we comply with those requests.