[header] How to stop cats from peeing on the front porch [title] Remove resources. [step] Remove food, water and other things that could be highly prized by visiting cats. This reduces the " value " of the porch and makes it less worth it to stake a territorial claim. 
[title] Use a motion-operated compressed air canister. [step] An ideal deterrent is a canister of compressed air that is activated by a motion sensor.