Don't want scientists talking about global warming? Easy! Just take away their money for going to conferences. That's what someone in the senate (rumored to be Oklahoma Republican Tom Coburn) did to NASA. The effects extend beyond muzzling the NASA scientists affected by this cut, as reduced conference registration is likely to hurt the societies that sponsor the conferences as well.

At least, they were less thorough in their slashing than they might have been: when this first came to light, it was thought that it would affect NASA contracts and grants as well as NASA employees, but that seems not to be true.

I ran a grant-sponsored workshop a number of years back; as far as I recall the grant money was split mostly between local arrangements (a hotel room to hold the conference in) and travel expenses for invited participants. There were no registration fees, and that model works only for quite small workshops. I don't have a convenient symposium budget online to point to, which would make this more concrete. But: as far as I know for the larger symposia I regularly attend (e.g. SODA, SoCG, Graph Drawing) the bulk of incoming money to a conference is from registration fees, with smaller amounts coming from proceedings sales (diminishing, these days now that everything is online) and sponsorships (from corporations and sometimes academic units; a conference may have half a dozen of these at $5k to$10k each). The bulk of expenses are the rooms and refreshments for the conference, with smaller but still significant amounts for proceedings printing, a tax by the sponsoring society for the conference, and invited speaker and program committee expenses. For conferences such as SODA that hire paid staff for the registration desks, that would be another expense.