I’ve mentioned this before on the forum earlier this year: Stopping users fiddling with Priority whilst needing the ModifyTickets right - #3 by GreenJimll. The ModifyTickets right is a rather blunt ACL tool.
I don’t think there’s a way round this in the straight RT distribution (if there is I’d like to hear it as it would be really helpful). You might be able to hack in your own new rights though (the benefit of open source if you’re a coder). Alternatively it might be something that you can pay Best Practical to implement for you.