Anglia installed a gate on the kitchen wall of the RFM which catering staff would shut & guard before Liverpool St to stop this practice on up peak time Norwich services.
Only time I remember issues with those boarding last minute down services via the only open door and then sitting with a standard ticket & not walking through until told was when we called at Shenfield to set down Southend Victoria passengers during engineering works but from what I gather passengers on this line a renowned for their attitude problems.
I remember this in Mark 3s on the WCML in the 1980s as well but presumed they were removed as a safety risk in the event of a collision or fire.