Can you use the cannon in pvp in runescape?

Yes, I believe you can. You can use a cannon at nearly any monster in Runescape. Not all of them, but most. It is best to find a world with not many people in it so you get more kills and so people don't get annoyed with you lol. :p

Almost any monstor below level 100 combat can be blasted by a cannon. I believe some bosses, including the Elemental chaos (276) KBD (276) Kalphite Queen (forget) Bork (226) and a few others can destroy cannons.

However if you want to use Barker Toads in replacement you can do that instead, since they have an HP value.