You could take the bike to a different shop, explain that the rear derailleur doesn't work properly and ask for a basic tune-up. It might be a quick fix as others have suggested. Rather than mess with limiting the gears, just get it working and use the gears that suit your riding. Leaving it on the middle chainring on the front, and shifting a couple in the back might suffice, but the others will be there when/if you need them.