Beheading is too serious, how about a simple \"kick me\" stuck to the back of the character? (does no damage, stalls them for 1 second)
The \"kick me\" sheet is stuck on a person for 10 minutes real life time after they speak in leet.
As for how a kick feels? Well maybe they screen can jitter just a little. (world view only text/chat box is unaffected.)
Edit: One more note, to make it easy to kick it might be as simple as picking up a crystal. Sadly, if someoone has enough time, then a person can potentially be stun locked for 10 minutes because of his leet speak.. maybe the time can be tuned down as 10 mins might be too extreme?