If my daughter is getting drunk and dangerous on money I'm giving her, I'm going to stop giving her money - not even a dime. If I gave her a car for college I'm going to take the car back.
If her grades are not up by the end of the semester (which was your agreement), I would not be paying for any more tuition/books either. I'm assuming tuition room and board have all been paid for already for this semester.
She can figure out how to pay up front or get loans next semester. If you are so inclined you can reimburse her that money - after the good grades are in.
ETA: I was once a drunk and dangerous daughter, and yes - my parents quit giving me money. I wasn't even mad at them. They paid for 4 semesters of college but my grades were fine.
Instead of feeling like the bad guy, or like you are giving up on her, re-frame it. You are being a parent helping her learn the skills that she is going to need for a successful life. That is the ultimate goal of parenting even if it isn't warm and fuzzy in the short term. That is the plus side if she is NOT an alcoholic. If she is an alcoholic it is more complicated but you may be saving her life by ending the enabling as early as possible.