Burning coal is a huge and easy win. Artisinal and small scale gold mining should be high on the list too, even though it's a much harder problem:

https://www.unep.org/globalmercurypartnership/what-we-do/art...

I'm skeptical that it's easier. On the numbers alone, artisanal and small scale gold mining (apparently) accounts for 15-20% of global gold production. But coal accounts for 35% of total electricity generation.

You do mean banning rather than burning, right?