What is the user banning function?

Removed

[kh99]

I don't think there's a single function that makes it quite that easy (well, there could be but I don't know of one). But you could look in inlinemod.php around line 1225, the section that starts like this:

		switch ($vbulletin->GPC['useraction'])
		{
			case 'ban':

It uses a user data manager to set the user's usergroup to the banned group, and also writes a record to the userban table.