How to make students learn prerequisites
I think you can't make students learn anything, and it would be especially difficult to make them learn something other than what you are teaching at the moment.  Still, I can think of some things to address the problem (many of which it sounds like you are already doing):


You can make sure that the unprepared students see early on that they are unprepared and liable to fail the course.  This might not motivate them to learn, but it is still a nice thing to do.
You can give exercises that require them to learn prerequisite notions incrementally, giving them hints on where to find information not covered in the lecture. Just don't expect them to learn more than they need for each problem.
At the end of the term, you can make sure not to pass the students who didn't learn the things you said they would need to know in order to pass.  (Otherwise you justify the very behavior that you complain about.)
You can pitch the course at a lower (but still nonzero) level.  In my opinion the only reason not to spend more time on the basics would be if there were a significant number of students who would be bored by this.  But maybe they would appreciate a review anyway.
Instead of changing the course, you could change the course description to try to deter more of the unprepared students.
If the students vary too widely in their backgrounds, you could try to create different versions of the course in future years.