Creepy people

Whenever I turn my laptop on my aim account just automatically signs on. For some reason recently I have been getting random n creepy IMs from people I don't even know. Some say really inappropriate things which i just ignore. I dont get why people like to be so dirty to others for no reason. While today I was doing my work and studying for my finals I get an IM from someone I don't know and the conversion goes like this...

sexysili05 4:23 pm
hey there!

rumaisa butt 4:31 pm
bye there

sexysili05 4:31 pm
hey whats up?

rumaisa butt 4:31 pm
who is this?

sexysili05 4:31 pm
just a lonely girl looking to chat you interested?

rumaisa butt 4:36 pm
uum not really im busy right now

sexysili05 4:36 pm
oh ok. so how are you today?

Did she not get it when I said I'm was busy? I don't want to be rude but they are some creepy people out there and what if this person is some creepy old guy pretending to be a girl. I was looking to see if there was any why to limit or block people on aim but haven't found it. It isn't only aim! I was on Skype one day with my friend when i was getting a video call from someone else. Not knowing that random people can video call you without being a friend, i picked up but closed it withing 5 seconds because i couldn't see their face. The person then messaged me on Skype asking why i closed it. People like this just annoy me and creep me out. How can you just add or just randomly what to chat with one online you don't even know.