There was a real spate of this following new cycle routes in the centre when car (and lorry) drivers treated it as parking.

What worked there was repeated reporting of each vehicle to Bristol Council parking services using the online form at https://www.bristol.gov.uk/parking/parking-service-contact-form.

Twitter can also be very successful, if you have an account and particularly where there is a photo. Include a poke at @bristolcouncil (and us as well @bristolcycling).