@YoloCraft in Xenforo 2 you go to -> Users -> Warnings -> then you click on the permanent warning and select -> move to usergroup (my usergroup)
That way when the user receives a permanent ban, it will automatically move them to the usergroup "banned". You will need to make sure this usergroup is created and then you will need to give it the user title "banned" and also some css for the username such as this:
CSS:
color: grey;
text-decoration: line-through;
This is what I do on Xenforo 2. I see that your targeting Xenforo 1.5 but the process should be fairly similar at least I believe it is.