Nothing. See: Mark Pincus & Zynga.

If you don't have a seat in the board room, assume you have a giant target on your back. As soon as the company has a reason, you can be diluted to nothing.

You can get a lawyer, but the proceeds are so uncertain, especially if you successfully sue. What do you get, 1% more of a company that has just lost an expensive lawsuit?

(Request: court records for a normal middle-class equity-owner in a small company fighting his rapid dilution, successfully or not.)

What prevents a company from not paying you the salary you're owed for the month?

What prevents a company from firing you because you're a woman, or because you're black?

The answer to your question, and these questions is the same: lawyers and lawsuits.

In California, firing without cause in order to deprive an employee of a cash out event is legally actionable.

How easy it is to prove is a different matter that I am completely unfamiliar with (it could be very easy or very hard).

That depends on the contract. Be sure to read the fine print.

Well once you get past your cliff you're vesting so they're yours if you want to exercise them.

Unjust enrichment.