Finally we have also spam in the trac, a lot! I think Michael is very busy to set invalid flag to fake tickets, sure this is an unwanted task

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Yes, this is not the appropriate board to speak about but found no better place.
In my opinion spamming content should be deleted as soon as possible not only for the content itself or to avoid confusion but primarily to counter the objectives of these people; they wants traffic from links and hits from serach engines, so we must cut down traffic as soon as possible an also avoid indexing from search engines. Spammers will find that theirs is a futile effort, then they migrate to sites subject to less control.
So simply setting the invalid flag don't fit the task.
I know deleting unwanted tickets is an hard hand work because the trac system never minded to do this. A plug-in exist to help in this job:
http://trac-hacks.org/wiki/TicketDeletePlugin.
Another plugin to help preventing spam:
http://trac.edgewall.org/wiki/SpamFilter.