I totally / absolutely disagree with you about...
No estoy de acuerdo en absoluto...
I cannot agree with you on this issue...
No puedo darte la razón...
You are mistaken if you think ...
Estás equivocado si...
You must be joking if you believe if...(inf)
Estás bromeando
You cannot be serious....
No puedes estar hablando en serio...
I completely reject Fatima's point of view...
Rechazo totalmente...
This does not seem to be the right way to deal with this problem..
No me parece la manera adecuada para tratar...