Like frosty said...make them over commit. Here's a log of an example:

[www.qhcf.net]

Also:
Energy drains (spell/scroll)
dispel to trip/cheapshot/bash
Catch them with no fly/protective shield. As a thief, I'd try to trip them every tick to see.
Muter can probably do a good job. Get a muter friend.
Lagging drive might help
Let them chase you for awhile and their spells might start to fall