You're going to have to give us a little more information than that: What build of bukkit are you running? Do you mean an ITEM or a block? What item are you trying to block? (some items are harder to block) Do you want it to be disabled in just one world or all worlds?
Install WorldGuard Open ../plugins/WorldGuard/worlds/<worldname>/blacklist.txt At the very bottom, place this: Code: [10,11] on-place=deny,log,notify,tell message=You are not allowed to place lava! Also, if you want, you can add ",kick" to the end of the on-place line and it will kick them with that message.
Another vote for Worldguard. Here are the relevant contents of my blacklist.txt: Code: # Deny lava buckets [lavabucket] on-use=deny,tell,notify,log # Deny Lava blocks [10,11] on-place=deny,tell,notify,log # Deny TNT [tnt] on-place=deny,tell,notify,log