It is a complicate situation and I'm sorry to know that you got to that
point. Dropping a class isn't a solution because the only person that would
"suffer" from this would be you and no one else (that could be a solution
only if a lot of people, more than 50%, would stop doing their assignment on
WebAssign at the same time, you can call this a boycott).
If I was you I would :
-- Send and email to every math teacher on the university talking about the
problem and a possible solution
-- Talk about this issue to other students in order to make them do the same
(sending the same email or signing a petition)
-- Send an email to WebAssign talking about your problem
But since you don't have any other solution, you need to do it, I feel
horrible for saying that but it is preferable for you, as a student, to do
this using a LiveCD than loosing your class (and the money by the same
occasion). A lot of people may not agree with me (and I don't agree with
myself either), but (for me) it is better than just saying "OK I Will do what
you want !".
I had the same problem with Cisco Packet Tracer (where there is no
alternative for doing what they want), but I am "discussing" about that and I
maybe found a solution, because the teachers can actually listen to the
students (not all, but it is better than no one at all).
The best you could do, is to find a professor that agrees with you and would
be likely to send an email do WebAssign with the same problem, that could
make WebAssign change a little bit the way they work.
Hope it helps.