I'm afraid I don't agree.
I agree with that.
I don't believe that.
I disagree entirely.
I'm not sure.
Rubbish!
I don't really agree.
You must be joking.
I don't think so.
I can't accept that
you have a point, but you’ve missed the point completely
I can’t possibly go along with that.
I'm afraid I think that's complete rubbish.
I can see your point, but
On the contrary, I
I'm afraid I tend to disagree.