Is this form of advertising in Clifton, Bristol, allowed?

I spotted two bicycles parked in Clifton, Bristol, over the weekend. Not an unusual sight, but attached to these bicycles were big yellow placards advertising an event at Ashton Gate.

Parked in prominent positions at the top of Park Street and in Clifton Village, it was a blatant way to get around advertising laws.

There is no way this should be allowed as it ruins the streetscape. But is there anything that can be done to prevent it happening in the future?