The main tool for this is negative keywords. A negative keyword is a word or phrase you add to your campaign to tell Google not to show your ad when someone searches for it. For example, if you sell premium software, you might add the word "free" as a negative keyword so your ad does not show to people looking for free tools. Beyond that, using phrase match or exact match keywords instead of broad match gives you tighter control over what triggers your ads. Do you review your search term reports on a regular schedule?