I usually say that as well as seeing them do a floor spot, I need to convince all the organisers that they are worth booking so could they email me details of .mp3/website/MySpace/YouTube etc. where we could all hear/see them and this would help us to make a decision.

Then when any email comes, I give my standard saved reply - whether we are going to book them or not - which reads:-

Thank you for your suggestion of a booking for the artist(s) concerned. They will be considered alongside all the others by those responsible for our programme. We will only get back to you if we have a positive response.
best wishes
Vic