Talk to them about dating violence. Let them know what is permissible and what isn't.
Role play. Ask them how they would get out of a given situation.
Let them know that you are there for them. If they need to call you at 3 am, you will answer the phone, you will come get them, you will not be there to judge ~ just to help.
Remind your teenager that they always have the right to say "no".
Talk to your teen about love. Tell them that jealousy is not a sign of love. Possessiveness is not a sign of love. Respect is the sign they are looking for in a partner.
Consider only allowing one on one dates after you have gotten to know who your teenager is dating.
If your teen has had an experience of dating violence and you are the one they have chosen to talk to, then listen. Do not judge. Use active listening skills.
Tell someone who can help. There are people trained to help in these situations. One hot line for domestic abuse or assault is 1-800-333-SAFE.