I witnessed an incident on a GLA-LCY last year where a guy in crutches had booked himself into 2C on an Embraer 190 which is an exit row. He was almost offloaded as he became aggressive and refused to comply with the CC request for him to move to empty 2B due to safety regulations (the usual "I always sit here when I fly" line from him). The Pilot backed up the CC and the guy moved seats (which had the bonus of meaning I had an empty seat beside me as I was in 2D). So it does go to show that some CC do try and enforce the rules.