Disinformation is "Information" which is purposefully false. Getting in to a fight about whether it is true or not just boosts its engagement. Most viral disinformation only spreads in the first place because of all the people sharing it to show how wrong it is.
Instead of helping it spread, just call the person who shared it "weird".
This works because people crave social acceptance. No-one wants to be the weird kid. By labelling this behaviour as 'weird', you side-step the "true or false" debate entirely and put pressure on the sharer to stop.
TL,DR: Engaging a lie makes it breed. Calling it weird kills it dead.
