I was testing it on my boomkin and the following toys will overwrite the form:
robo-gnomebulator, mr smites compass, jewel of hellfire, krastinovs bag of horrors, leyaras locket, cursed feather of ikzan, orb of the sindorei, coin of many faces, mark of the ashtoungue, moonfangs paw
Also, the Spectral grog (buy in auction house or farm in Draenor) will overwrite the boomkin form an it lasts 1 hour.