STYLETHREAD -- LET'S TALK SHOP!

This might belong in the relationship section...warning, it's long.

I have an old friend of 9 years, "M" who I haven't talked to since July 2007.  We haven't been close in the majority of 9 years because she's rather hard to be around for long.  She is a little younger than me, which when she was 19, was the reason I excused this behavior, but she's an energy vampire.  She has a history of trying to "sell me out" without success, as I am pretty much a discloser and I don't hide things about myself in general.  Our friendship has been on and off, sometimes I hadn't heard from her in a year.  I never sought her out, she usually called upon me when she was on the rebound after losing another new group of friends or was having relationship problems.  I never really understood what she liked about me, but I guess I was the one person who was honest with her and always had time for her if she was in need of a friend.  She even called me her best friend.  I never ever referred to her as my best, but it felt nice that she thought so high (supposedly) of me.  M was a weird friend whom my other friends tolerated but often questioned why I hung out with her, some even said I should watch out for her.  The majority of my really good friends aren't local, I am kind of isolated where I live and since M is local, I felt a certain attachment I think and I treated her like a little sister who just didn't know the error of her ways with me, and received unconditional forgiveness.

Last Spring, I found her on myspace after we had lost touch while I was busy with my 2nd baby.  Later, she was feeling nostalgic and wanted to start hanging out again so I let her back in to my life.  It was okay for a month or so, as she can be fun to be around in moderation.  But a little later in the summer she had revealed to me (only because she sort of got told-upon by a friend of mine who finally decided to tell me about it) that she had betrayed me in a substantial way.  Luckily for my sanity, the confession didn't affect me with heavy gravity because it happened long ago in 2002 or 2003.  However, the incident was bad enough that I was wounded, annoyed and confused and wondering how in the world I had ever let a person like this be my friend.  I felt like this girl was entirely untrustworthy, like maybe my intuition was telling me that she was out to get me and deep down got pleasure out of seeing me be hurt by something.  I don't know that she does this conciously, but I think she is a natural friendship-sabatuer.

I wanted to be done with her last Fall after I tried to call her to get back a pair of sweatpants she had borrowed (she constantly raids friends closets!).  I called three times, in the last message I told her I was disappointed but would not be calling back.  I didn't call again (though I did get the pants back through a different friend).  I am sure she felt uncomfortable after revealing the weird incident in which she betrayed me years before and that's why she didn't call me back, but I saw this sweatpants for ransom incident as a great opportunity to be done with her in a definitive way.

Anyway, it's almost been a year ago and she's still on my myspace friendlist and I have noticed that some of MY old friends who had met her with me years ago when I was single have added her.  They probably think M and I are still close, they don't even know her well, as they live in other places, so it's probably stupid in reality, but I don't want people to think I hang out with her.  I have no desire to say disparaging things about her, explain what she has done to me and why I don't like her, but I don't want her to be a part of my life and if she is "myspace friends" with my real friends, that's kind of a hard place for me. 

Is it totally immature to delete her now?  Should I explain to those of my friends, who haven't yet heard the story about her, that we aren't friends anymore?  I am 32 and part of me feels like even worrying about this is really stupid because myspace shouldn't matter.  help?