Please don't throw a rag, or anything else into a spinning prop. The prop can break throwing fragments all over the place, and if you happen to be in the wrong spot, at you. Even if it doesn't immediately break the prop, it's applying severe stress to it, which could cause it to fail at some other time.

If closing the throttle and throttle trim doesn't kill the engine, you need to re-adjust the linkage. Throttle all the way back, but trim forward should give you a decent idle. Pulling trim all the way back should close the carb.

Worst case, (carefully) squeeze the line to the carb closed and hold it. The engine will starve and quit within a few seconds.

Jetts' advice for starting is good. You haven't been playing with needle valves have you? Covering the carb (or muffler) to prime the engine normally shouldn't have to be done more than once per tank. If you're doing it repeatedly, you may be flooding the engine.

If it's starting, running for a couple seconds, then quitting; I'm inclined to think your throttle needs to be open a bit more.