Hi Mark, great question. We've encountered this before to an extent.
We used to use #eventschat for our events (promoting & audience engagement) and it used to get a pretty big reach, trending in London a couple of times. I noticed that after the event had finished the hashtag was being spammed by a Russian dating site, but since the event was over and they were pretty much the only activity we didn't do anything about/think anymore about it.
But now you've brought it up, what can you actually do? Apart from asking them to stop using it you're pretty much powerless no? And in our particular case I imagine it was just a bot that latched onto high impression hashtags, so they wouldn't have responded.
Would love to hear from someone who has had this issue more seriously than me, and how they dealt with it?