You can use a metal spoon to keep it down.
Contrary to popular belief, metal in a microwave isn't 100% a no go. It is just easier to present it that way to avoid confusion as few people know the workings of a microwave and the consequences thereof.
While it generally is discouraged to leave metal objects in the microwave, the problem isn't the metal itself. Rather that metal will reflect microwaves and not absorb them to increase temperature. You already have metal walls in your oven for this reason. They help reflect the microwaves so eventually they will hit the food or liquid and me absorbed. If you have a mass of something containing water (i.e. pretty much anything organic) a little extra metal should not cause any problems. Especially if it is partially or fully submerged in the liquid.
I found another source of this by Google. I don't know the credentials of this site, so take it with a grain of salt. It does however look like he knows what he is talking about.
Note: Please don't do this haphazardly and carelessly. Find another source than me saying this and read a little on why this works. A microwave oven is a powerful tool capable of concentrating about a kilowatt of energy in a small area. Treat it with respect.