Is multisig burning possible? Has anyone tried it? I want to create a dapp where visitors can burn tokens associated with my service. Looking for examples. 
I think this would be policy specific …
For example not all minting policies will even allow burning (before slot, etc) and the policy owner would also have to provide a witness signature allowing your service to facilitate this.
In short, I do not know if it is currently feasible to have a third party “unminting” and of course it is not possible to “burn” ADA not that anyone would want to anyway …