Well, there are a few ways.
First, if there's an antimagic zone anywhere, go stand in that for a while. There is one in the basement, in the Enchantment classes somewhere, unless they moved it. If she enters it, she should disappear unless she isn't summoned.
If she isn't summoned, that can only mean she was Called. Calling is different than summoning. It brings a creature here on a semi-permanent basis for extended tasks. Normally, a bargain has to be made for a calling to work. Summoning puts chains on the summoned creature, while callings require ceremony and diplomacy.
If you Called her, she may be around for a long time. Try stopping and asking her what her business is, and if she is unwilling to make a deal, that you have no use for her.
Reject any and every deal she offers. Either she'll give up and un-Call herself, or you might find out other stuff, like... maybe someone else called her and made her follow you?
Whatever you do: Don't fuck her, don't kiss her, don't hug her, don't even hold her hand. You won't even realize your life force is being drained. Remain strong, and attack her if all else fails. Succubi are demons of stealth and trickery, and cannot stand up to a full assault.
Good luck, kid.