Are you 15 with a newborn and don't know how to stop having sex?

Stay at home and raise the child you have.

Just tell the guy or girl you are sleeping with that you don't want to sleep with him/her because you are busy taking care of his/her child.

Just Say NO! Or see if you can report the father for statutory rape. That will get him out of your hair.

ERROR: You must spread some Reputation around before giving it to martinizing2 again.
What do you mean how to stop having sex?

Do you just not know how to say no? Or are you in a situation where you're being forced to have sex?

If you're being forced into sex that's rape and you need to call the police.

If it's a matter of you just not knowing when or how to say no, then you just need to get used to it. "NO." and don't put yourself in positions where sex is even an option if you can't say no.