I was owed billable hours from Stanford University but refused to do any further work until the outstanding invoice was paid. Their accounts payable department didn't want to, so I had to tell the client "sorry" and walked away. It's really weird because I didn't have a problem in the near past then and was even an FTE at one point. I'm unwilling to work for free or for imaginary, low liquidation preference equity of a worthless startup.

Ordinarily, I would sign master agreements and set PO terms up-front. Typically, the better customers would agree to very strict requirements / objectives for a particular time period for a specific price. Any deviations would require negotiation. Hourly is fine too but there must be regular milestone deliveries so that there's demonstrable value for money being conveyed rather than an appearance of a milking-oriented consultant. Expectations must always be managed.