When should students stop taking the ACT?
This depends on the student and their score.
I do not think a student should take the ACT during their senior year unless colleges they are considering have told them that if they achieve a certain score then they will have a better chance of getting accepted or they would get a scholarship offer increase.
One year I had a senior in my spring class and she was taking the April ACT because her number one choice told her that if she scored one more composite point on her ACT they would increase her scholarship $20,000.
I’ve had many students negotiate scholarship offers just like that with schools.
If a higher score means more money, then do it!