If you can prove that the other shareholders are plundering corporate assets, you can sue them on behalf of the corporation for breach of fiduciary, and/or for conversion/civil theft. If you win, you could report their actions to law enforcement and they could be prosecuted for embezzlement.
Unfortunately, law enforcement will not generally get involved with this sort of white-collar crime until after a successful civil case is proven, because, frankly, it's cheaper, since the government can use your evidence to help prove its case.
So, as you say, you will simply have to threaten your co-owners with legal action, unless they voluntarily account for and restore their ill-gotten gains, and if they refuse, then you will have to sue.
For a civil litigation attorney referral, see: http://www.abanet.org/legalservices/lris/directory/main.cfm?id=FL
Hope this helps.
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