She won't get out of our lives

I kinda need advice.
I drive myself to the point of a breakdown with worrying about my fiancé and his other friends. I guess, it’s jealousy in a way.
We had a few problems in the way that his friends were convincing him to cheat on me (he never did) but it settled doubts in my mind until he cut them out of his life because I meant more. This meant a lot to me.
But then my friends started causing problems in the fact they just didn’t like him. This caused a few months of arguing between us all but thankfully its all sorted and we’re all friends again.
But one girl, Becky, is still in the picture because her boyfriend is Marcus’ good friend. The reason this is so bad is that Becky has made no secret that she fancies the pants of my fiancé. She always finds excuses to touch him up, have play fights and hug him. And whenever I and Marcus have cross words with each other, she’s always a shoulder to cry on, and Marcus has told me that on more than one occasion, she’s tried to convince him to end it with me.
This isn’t the first time though. All through high school, she messed up every relationship I had. Although it was always because she ‘was trying to help’.
When I finally snapped and told her to just stay away from us, she told me to let Marcus off his leash for once because he would rather hang out with her sometimes as it’s less pressure to be all lovey dovey.
But that’s not true. Marcus doesn’t even like her, in face, he can’t stand her and it makes him angry that she’s getting to me so much. But he wants to keep the peace so he doesn’t say anything.
Am I wrong in that I just want her out of my life for good? Or am I being over the top?
I just don’t trust her anywhere near Marcus and all my friends at college have agreed as they know exactly what’s she like.
I just don’t know what to do. =(

(Sorry it’s so long. I needed to vent)

I don't think you're wrong in wanting her out of your lifes.
If i were you, first i'd try and have a civil conversation with her, as hard as it may be, about the reasons why you feel like you do, and why you don't trust her, and see where it goes from there.
If that doesn't work and she doesn't get her act together i'd tell her that Marcus doesn't even like her along with the reasons why and that she needs to stop messing with your life and get her own sorted out and to stay away from you both.

I think i love you, lol.
Thankyou! You right, it's really hard to be civil when i just wanna run the other way every time i see her. But i guess its worth a try?
Thankyou x

Lol, no problem, and you're welcome.

this may seem harsh but civilized conversations wont work seriously just crush her spirit right away and tell her how marcus hates her and every1 else thinks she's an annoying little b*tch just make crap up as you go along and hopefully she'll get so pissed she'll either just leave you alone or do something very bad which will deffinatly get marcus to stop communicating with her at all

Wow, thanks everyone. I didn't expect this much advice and it's all been a big help to know i'm not imagining all this.
I was upset last night over something unrelated and as me and Marcus were walking to the bus stop, she showed up out of nowhere! (I can freely admit that i thought she was stalking me, lol) and when she stopped to talk i sighed really loudly and said in her earshot, "Oh for gods sake, not her!" She raised her eyebrows and touched Marcus's arm and went, "Is everything ok?" I just stormed away with the parting shot, "When your finished talking to it, come get me."
To my complete delight, Marcus cme straight after me without saying anything to her.
I've not seen her yet at college (Where i am now) but it's just a matter of time.
We'll see what happens then i guess.
Thanks for your support everyone. Much appreciated!!!

talking should help, yeah.
Really good advice from all the others!