So, i did some changes for the votings and set stricter rules. Somebody set up an automatic system to submit downratings. All of his/her votes were now deleted.
I really don't know why downraters are spending so much time and effort to manipulate votes. All IP addresses are recorded and it is only a single mouseclick for me to delete such fake votes.
As always I will keep an eye on my database and as soon as something another pops up, I will make corrections.
I dont think that I need to change the system so that only registered users can vote because of 2-3 black sheeps. We want to have as many votes as possible to get also meaningfulness numbers.