Actually Jim I thought that I had submitted an answer to dealing with persistently bad performers. At every club with which I have been connected (4 in 42 years)the MC has always had licence to put on who he/she deems suitable and in which order and if it goes tits up then they have some explaining to do; believe me I have been on the wrong end a time or two! However if a persistently bad performer finds themselves not being asked to participate on a regular basis then their own sensitivity should tell them that something is amiss. Unfortunatley as I said earlier the lesser the talent the greater the ego and if the club organiser/mc is approached by the wounded would be performer then it is their duty to inform them as gently and diplomatically as possible that they are not of the standard that the club aims to present. More than often this will result in a torrent of abuse aimed at both the club and it's residents but at least the would be performer has got the message.
The above action was applied to my mate and I many years ago at Birtley club (read Pete Wood's just published"The Elliotts of Birtley" for a full description) and it worked for both of us.