Based on the episode rules Kenny and Spenny agreed to, no one cheated. It could be argued by Spenny's 'spirit of the competition' that Kenny cheated twice.

The first time by getting his man wad injected with a numbing agent. This saw Kenny able to avoid popping wood when Bianca, Spenny's super hot cousin, gave him a lap dance.

The second time Kenny cheated is more about him bringing Spenny unbridled pleasure, by letting him finally get a lap dance by a walking meatlocker named Hugo. Spenny wanted a woman, likely resulting in him cheating and not getting a boner, as evidence clearly shown throughout the series has him loving the spam noodles.

There was a third possible attempt to cheat by Kenny, where he began the dastardly plot of giving Spencer an enema. This attempt however was foiled by Spenny and lead to one of the funniest moments in the series' history; mutual wedgie.