TwitterSpam? Hashtag Hijacking?

Help prevent/discourage inappropriate Tweeters! #FridayLive "No good deed goes unpunished?"  
Join TLT Group this Friday 5/13 2pm EDT "Twitter: Forbid It, Ignore It, or Use It? Insight and Implementation" Derek Bruff, Vanderbilt University - FridayLive!  Free

We've begun using #FridayLive as a Twitter hashtag to encourage people to exchange info before, during, and after our free weekly usually very interactive online sessions [Fridays, 2pm EDT tlt.gs/frlv].  This began as an experiment, and we're still exploring variations with the hope of increasing participation, increasing exchange of useful info, extending the benefits of the activity, learning how Twitter can enhance or distract, etc.  Apparently we have begun to "succeed" to the point of attracting a new kind of unwanted attention and participation:  i.e., some people have begun adding irrelevant, self-serving, or unrelated commercial Tweets to this stream.  Any recommendations about how we can continue our experiment while reducing this irritating element?

"To Twitter users, the hashtag isn't a new concept: attend an event and use a hashtag with any given word or phrase to organize tweets from the said event. For example, if you were at a big gathering of pizza lovers, you might use the hashtag <#pizza> in your Twitter stream. Fellow attendees of that event, along with observers, can follow discussions by simply following the #pizza designation. This practice comes in handy during large conferences, blogger events or live chats.

"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. ... 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'. " From "The Latest in Digital Spam: Twitter Hashtag Hijacking" in The Daily Blagica