How to be nice without being taken advantage of?

I think it’s good to be nice to people. However, there are people out there that take advantage of those who are nice. Some e.g.:

1. (School setting) Guy comes up to you to ask you a question about a homework problem. You answer. From that point on, the guy comes to you whenever he has a question about homework. You want to help but:
—>A) You are busy yourself and don’t want to be constantly peppered with questions AND
—>B) You wonder if he is just slacking off and relying on you (esp. if the guy has a rep for slacking) AND
—>C) The guy does not invite you to anything fun, ever. He just comes to you when he needs something

2. (Group project) Group of four or five. Three are slackers. They expect you and possibly the remaining guy to do the work because they think you are “smart.” You can:
—>A) Do everything while the slackers chit-chat OR
—>B) Try to work things out with the slackers, who would not budge OR
—>C) Not do the work and everyone in the group fails

So, how can someone be nice and not be taken advantage of?