Some of these will be used individually depending on the situation, a combination, or all of them together situation dictating....
Treat them nice, treat them cool, treat them direct, treat them like a child.
1) Treat them nice: You say what you need/should/supposed to say, but in a simply friendly way. You can be firm AND have a smile at the same time... sounds weird, BUT, ego's abound, so it offer's a less confrontational situation. Especially if they are with their girlfriend (they gotta be a "big-man" dontcha-know). Show respect... get respect.
2) Treat them cool: stay CALM if they give you a hard time. Try to "win them over" showing them you are actually on their side... "Hey, I know, you found a good spot to sit and I wouldn't want to have move over either, but...." "Hey, it looks like you guys are having a really great time over here, I wish I could join in. But, listen, if you don't mind guys, some of the people over there were complaining about 'x' 'y' 'z'. I need to ask for you turn it down a few notches." If anyone ever escalates, you never show any ruffled feathers. A calm demeanor can de-escalate ALOT of situations.
3) Treat them direct: it's simple... people don't think- they really don't, it's amazing. Not to go into details here, but as a leader, I learned so much about people simply when I was a parking lot attendant. You need to be specific with the people of what you are looking for/want from them.
4) Treat them like a child: it actually gives them a false sense of "control." What I do with my 6 year old daughter- I give her a choice on something, BUT, either situation is something is what I want. "Well, I can't have you guys hang out in this area right now, but, you could go either over to 'A' or over to 'B'". Again, this gives them the feeling they are still in control and not being told what to do.