Protesting, public pressure in other ways, pressure through other representatives in Congress. Also the same to try and get the voting system changed so minority parties can have more effect, bending the major ones to have to talk about issues that for now are easy to avoid (the both sides, even if that's not entirely true). Another factor is lobbying, that needs to be restricted so large entities like corporations can't basically buy loyalty.

I would point out that any vote, even for Stein, is unconditional, so there's no way to avoid that. To make politicians keep their policy the public has to be engaged past the election.

Even if all of that is debatable, my main point is that a vote for Stein won't get any change. One of the two choices that can win the election has some chance, even if small. Whether that be from citizen pressure or them getting the power of office and doing things themselves.