How many missions, rovers and experiments would it take to convince people that there was never any life on Mars? Well, it's impossible to prove a negative conclusively like that, but we could reach a point some time in the future where it is more reasonable to believe that there is not, and never was, any life on Mars. We keep going back with more and more specialised equipment designed to find life that a lot of people are convinced was there. But every mission that fails to find signs of life decreases the possibility that it was ever there. At some point in the future, that hope is going to stop being reasonable. Some would say it already is. If you want to go back to Mars after that, you're going to need a different argument.
Mokalus of Borg
PS - Colonisation seems like a weak answer.
PPS - It's more expensive than fixing Earth.