running workshops is good but running workshops successfully requires skill and diplomacy and tact.
firstly singers should not be told they must sing in a certain way[stylistically],
workshops should concentrate upon improving technique,suggestions can be made,directing the singer to recordings of differing singers that the person might benefit from listening to.,and then let the person chhose their own direction,suggestions can be made that a singer might be more suited to singing shanties or whatever,but that is about as far as you should go.
the singer has to make their own decisions,telling people what to/ or how to sing will often be counter productive and is bad manners.
it is also bad manners,to say to a performer while they are on stage: we dont allow political songs here,or you must only sing songs from your own culture or in your own accent[ Ibelieve this is what happened to Lisa Turner,a fine blues singer at the ballad and blues club,or was it Maccoll/Seegers singers club].
A club may have rules,but the time to sort that out is not on stage,but during the break or afterwards.