What do you do when a Guinea pig doesn't want another guinea pig on their territory?

If they are male and female, and you do not want the female to get pregnant, then IMMEDIATELY take the male out of the cage and put him in a different cage for a few hours. If you WANT the female to get pregnant, then just leave them alone.... And if they are both males, then it's trying to show the other one that it is dominant. If there are two males and one female, GET THE FEMALE OUT!!!!!!!!