Just make it a habit to clear bricks before your final save. Only keep what builds you want. Anybody can save their own blocks, so there is no reason for them to complain (their blocks would become your blocks anyway, once you loaded after you saved, making you have to trust them if they wanted to continue).
As long as you clear people's blocks, there will be NO spam. Just be sure to do this before your final save.
It also helps to spy on people every now and then to see what they are up to. If it looks like spam, let them continue until they are bored, then just clear their blocks when they leave, or if their spam becomes a clear problem.
Hopefully Badspot will update the game and make it so that it remembers who block's are who's even after you loaded a previous save. This would make it so that spam is no longer a problem.