I've counted before nothing really happens to the spoiled papers. You just think 'ugh, a spoiled paper' and chuck it in the spoiled paper pile.
The candidates/agents can review them if they want to, and if any mark is just in one box next to a candidate they can argue that it's a vote for that person. So don't put your protest just in one box or it could end up as a vote.
To be honest though, it's a waste of time, nobody really pays much attention to them.