You are fortunate Snail, and I think we help run similar clubs, usually the lesser the talent the greater the aggression if this particular person is refused a spot.
I may have mentioned this elsewhere but at a club I help run some thirty odd years ago we had a lady, who was a good friend of us all, who turned up one week with her guitar. Naturally we gave her a spot and it was quite obvious to all, except her, that she had nowhere near mastered her instrument. She came a few months later with her guitar and the results were the same; this became more frequent until members of the audience began complaining to us. So on one occasion she was deliberately passed over for a turn; rather than sending out the message she was waiting for me to open up the next week so that she would be sure of a spot, I explained that we had a guest but I'd see what I could do thinking that I was on pretty safe ground. The guest didn't turn up but I still missed her out and at the end of the night she demanded to know why and I had to explain as gently and politely as I could that she was nowhere near the standard to play in public where an audience is paying good money and expects something good in return. She really wwent off on one at me saying how far ahead of unaccompnied singers she was and much better guitarist than some of the bluesmen (we had two of the best in the North as regulars) who "just strum" she was. Plus the last time that she played at our club the room was dark and she couldn't see the notes.
After that, plus a letter to my home from her husband saying pretty well the same thing only in a more temperate manner, she never came back to the club and certainly never spoke to me again prior to me leaving the area.