That's a much more reasonable question, but not a terrifically interesting one; libertarians would enforce fraud law the same way everybody else does, by a combination of law enforcement (criminal law) and litigation (civil law). You don't see much of a push among libertarians to cut the enforcement of fraud laws. To some extent criminal law can be made much more efficient, by focusing law enforcement on things that actually harm people. If the immense cost of the War on Drugs (etc.) was gotten rid of, there would be more resources to go around.

(Personally, I'm not convinced click-through fraud is a particularly high law enforcement priority; there are technical means of combating it (as well as other Internet perils), and it's easily avoided by education (which is provided free by other netizens)).