The Latest in Digital Spam: Twitter Hashtag Hijacking

Event organizers are taking hashtags to another level, in the form of displaying a large screen at an event, keeping track of the live hashtag conversation for all to see. The image in this post comes from the Le Web conference in Paris. In the last month, I've experienced or seen six separate examples of Hashtag Hijacking: where someone deliberately uses an event's hashtag to have their tweets (often disgruntled) show up and 'crash the party'.
Example at the Pizza Event:
"I hate pizza in general, why have a party about it?" #pizza
"I'm having a party at 123 Maple Lane! Leave that party and come to mine!" #pizza
"*(&(*&(*&(*&(*&(*&*(&(*_)(_)()_!!!!!" #pizza
I don't know the answer to stop these practices, but I do see them occuring more often - and it's unfortunate. The hashtag was meant to connect people on one topic of discussion, but is now being used to disrupt that conversation and draw attention to the Hijacker.
Have you experienced this lately? How have you mitigated the situation?