Laura
Why not letting the user decide?
If he/she only want to leave and have his/her account deleted,his/her posts should be left,but if he/she asks to delete it,or delete it himself,it should be allowed.
Even if I agree,it can make some topics harder to read,and understand.
Angelina if you notice that a user who is no longer in the forum has previously deleted his posts, perhaps you could give a small hint so that nobody thinks so Huh?
Yes,in such case,it should be told that some posts in this thread has been deleted.