I think if it has been a long standing practice to allow their use it is difficult to call a halt to the practice overnight, but I think a campaign of 'discouragement'by drawing attention to is is in order - a simple statement that you are intending to try and phase it out.
I noticed on a London singaround website a gentle slap on the wrist to a couple of regulars - not unpleasant enough to give offence.
The ideal would be to offer help to singers who wished to develop - often a two way street in drawing in volunteers.
I have never understood the active support for the practice - do people actually believe that those who persist are incapable of learning songs - are the songs not worth the effort?
Beyond me
Jim Carroll