Contrary to popular belief it's actually in the unions interest to have a certain level of FO contractors as it reduces the risk of anyone on the seniority list being chopped when the next downturn comes.
Not sure about the reasoning here but you can't have the cake and eat it.
If the full time pilots don't want contractors then they should instruct their union to do something about it. If not, get on with your jobs and accept that sometimes you fly with contract colleagues, sometimes you don't.