Post Limit Code, anyone?

Hello guys,

anyone knows a code to limit the post of a user in the same topic. so he/she cannot bump again and again? i really need this to stop the spammers coz they just want to make their post count higher. TIA!!!

by the way i use punbb and my site is http://pep9.nstars.org/

if this cant be done how about posting of minimum of 20 characters only? so no one can post a message like "up!".