Clingy friend.

One of my friends is really clingy and its just getting plain annoying! Today, I'm just spending the day with my boyfriend and here's what happened on msn:

Emma: Hey Lara!
Me: Hi...
Emma: Can I come over today? We can watch the MCR thing, I'm a huge fan!
Me: Err, you haven't heard any of their music, how are you a fan?
Emma: I just am! So Anyways, can I come over?
Me: No, Sorry, Jonni's over and we're just gonna hang out for a while.
Emma: PLEASE! Oh, and can I sleep over?
Me (getting a bit annoyed): Em, I said no, I told you, Jonni's over.
Emma: PLEASE! (then she added about 100 'e's on the end.)
Me: No! I'm busy.
Emma: But you see Jonni every day!
Me: No I don't, he has work remember and while he's off I'm spending time with him, I see you about every single day!

And then she signed out. It's getting on my nerves, whenever one of my other friends hang out with me while she's there she gets all bitchy and starts arguements with them!
She's just too clingy! I like being by myself some days and she won't let that happen!
What can I do to get her to back off?

xoxo