Ways to temporarily quiet a muffler on a 99 Suzuki RM 125?

I would see if the packing has been removed. Your pipe may also have a quiet baffle avalable. I know my MRD pipe came with one. The first thing I did when I got the bike from the previous owner was to put it back in. Loud pipes get motorcycles kicked out of great riding spots, and usually all the noise doesn't come with much performance increase anyway. The DNR can be a PITA but noise limits are good for everyone involved.