Need some advice

Alright, I leave my house for a day or two (or several months for that matter) and my dad screws things up on the computer, Porn, Yahoo messenger with freaks so too keep my parents marriage, how do I keep him from doing this? Blocking Yahoo messenger and nearly all porn sites... I want to know, I dont care if I gotta put secure lockdown on my pc, I will do it... I need him to quit screwing my stuff up and he denys it.... so any help would be greatly appreciated! Cheers, Gunny