For the group you want to have perms, dont do anything.
For the group you want it to be blocked for, give them a negative permission node modifyworld.blocks.interact.x(insertRMFurnaceIDHere)
If you were using PEX:
Groups:
Deprived:
permissions:
- modifyworld.*
- -modifyworld.blocks.interact.x(****)
Privileged:
permissions:
- modifyworld.*
Where the **** is you need to put the rm furnace ID. Dont forget the x and the parentheses!
p.s. Just going off of memory but im pretty sure this is correct :)