I recently started a relationship with this girl and her ex has lately been really interested in her and trying to get her back. He's a complete AFC and he's not getting anywhere, we're past the point to both be against him. She's mocking him all the time. Anyways, last night she told me he had been disturbing her. Calling her at 3am and messaging her until she deleted him from facebook. She wants my help to get to disappear from her life. So how do I proceed, I mean, I can easily smash him because I'm way bigger and stronger that skinny clown. Should I do that or just have us all three meet up, have her say she doesn't want him in her life while behind my shoulder and then I step in and threaten him in some way? What would your approach be, guys? I'm in unknown waters with this relationship thing so I'm seeking advice.

Thank you for your time.

Do not threaten him. At least not right away. If he calls your gf, you pick up the phone and calmly (!) explain that he should refrain from contacting her in any way. You might even want to inform yourself about the legal measures you could take and illustrate these to him. If he throws a tantrum, invite him to come over or meet somewhere where you can discuss things, the three of you. Make sure you NEVER sound angry. Be polite, but firm.
In case he gets angry or even physical at the meeting, stop him immediately, but do not attack him. Tell him to leave or you see yourself forced to take further measures. If he does attack you, you may want to hit back. But be aware that you also might face legal consequences if you do so.

IF SHE ASKED YOU TO HELP WITH THIS:

I would call him personally, with her present so she can hear you on the phone. Phone call is short and sweet, say the same thing if he answers or if he doesn't and you have to leave a message.

Hey, XXXXXXX, it's VHOU812. Listen, I've heard that you are coming on a little strong to my woman XXXXXX. I really don't want to be the bad guy here, but we both would appreciate it if you could just let it go and move on with your life. I expect that this will be the last time we have to have this conversation. Take care and good luck.

End of conversation.

Even if he persists, she'll think you're the knight in shining armor she always wanted. If he does persist and she continues to ask for your involvement, the most effective tactic I'm aware of to deter people is to make it such that contacting her is not in their interests. Yeah, if you can kick his ass and don't care about legal ramifications of doing so at all, then go that route. I've found it to be more effective to get embarrassing dirt on him from her, and let him know that you know some of those things, and you're willing to share them with the world if he doesn't leave you two alone. Conversations like this in private let him decide if she's worth damaging his own image. 90% of the time they go away, because they realize that even if they win her back, their life sucks a bit because people know things about them they wanted hidden in the closet.