Are you sure you have manual a/c and not automatic climate control. I do not know much about these systems, but in my experience the a/c will come on automatically if you activate certain procedures, like windscreen clearance. I find the best solution is to just turn everything off, and start again.
The manual aircon will always come on if you switch to re-circulate. It is a nuisance when all you want to do is avoid lungfulls of exhaust fumes. Otherwise I am stuck for any conditions which have brought mine into play without asking.