Sometimes you have to be blunt and do as I did - ie ask them if they would work for minimum wage ?
Most of the time you have to gently coerce them in to your way of thinking.
The biggest problem is right now, there are some who WILL work for peanuts.
These weekend warriors are destroying the business, by giving many the impression the musicians can live on fresh air and that music has no true value.
That's why I got out of the computer soundtrack business back in 1996 when the industry was changing over from programmer/musicians who could write their own assemble language drivers and supply music & sound effects to run at chip level across multiple formats (my speciality) to just supplying music in CD Audio format.
I'd been earning a decent living at the former for quite a few years, but as soon as musicians could supply their music in CD format many software development companies hired in-house musicians on a pittance, some of whom even took their own keyboards into the office
It was time to reinvent myself