How to ruin someone is a pretty open ended question. It depends on what you want to ruin about the person, whether it is their reputation or their life. Maybe it's about ruining their credit. Maybe it's a question of ruining their self esteem. It all comes down to getting to know the person first. Try accepting the person instead of ruining them.
It's not a very polite thing to ruin somebody, and you should never do anything illegal, but sometimes revenge is good to have. The best possible way to ruin someone is probably to deeply humiliate them, I would personally suggest sleeping with their significant other and then constantly reminding them how lame they were in high school.