I forgot an update to this. I took note of some of the replies here that suggested I was being too strict or too paranoid. I had a good think and realised I'm a confident guy and I have every faith that my girlfriend loves me and wants me more than anyone else.
I also realised that a girlfriend is not faithful because her boyfriend is cutting off her options, but because she WANTS to be faithful to him.
So I spoke to her again and told her I don't mind if she talks to exes or hangs out with them. She instantly started telling me how she never wants to see any of her exes and kept going on about how she doesn't care about anyone in her life except me. I assured her I'm happy that she is on good terms with her exes and said if she had a connection with them it would be a shame to lose it. Almost as though I was pushing her to him.
She started pleading with me to understand how much she loves me and how she doesn't care about any other men and so on. She said because it caused some problems between us it's not worth it and she's cut all contact with this guy. She admitted a lot of things to me too including how she fantasises about us being married (another issue entirely that one).
And you know what? My paranoia is gone and I don't have the slightest worry about her cheating on me now.
I remember reading somewhere on this forum that when I girl disrespects you or does something you dislike, treat it with coolness. Instead of getting angry, upset or making threats, you should act as though her actions are slightly offputting. I think this is the vibe she was getting from me and now I feel like I'm back in control.
Call it mindgames if you like, but I like to think of it as just knowing how to handle a situation and keep level headed and I genuinely felt confident enough to say these things and mean them too.