Here's the situation:

I recently approached a girl in a bar, started talking, everything was going well. Then suddenly a guy she knew approached, and would not leave us alone. I tried to connect with her one on one but this guy kept butting in making me lose my focus. As he went off to the bathroom, I asked her how she knew him, she said that they are just friends. I then tried to isolate her to the corner of the bar, but he kept following us and putting me off track. It seemed like he was wary of my attempts at getting to know her, and engaging in a higher level; so much so that he would interrupt me while I was in mid-sentence.

Is there any way of dealing with these guys? Or should I try to move on and focus on other girls free of these guys.

You should try to get on the same page with her. You gotta see what kind of relationship they have before you do anything though. Like if he walked away start negging him while he's not around and you WILL get her true feelings about him.
If she is cool with him she will defend him when he's not around.
Example:
You: Damn man why don't he just hold your purse while hes at it
Her: (Not laughing) Yea thats Mike we been cool since high school.

If she doesn't like his presence either, she'll probably agree with you.
Example:
You: Jeez if he was any more attached to your ass he'd qualify as a proctologist !
Her: haha! Yea I know he's creepy
The goal is to get on the same plane as her.

I would just make jokes about them being together.
The girl will be starting to get curious what your intentions are. Display's in some way's that you're risking to lose her. High value.

And otherwise you just ask her number and continue this conversation another time when he is not around.