Pragmatism and protest are not opposites. You can do both, and it's better if you make your protest as effective as possible, rather than just doing a lazy protest on election day.
A vote is a choice to hire one of the people who you can hire. If you want to undermine the power of the two parties, the way to do it is the way the Tea Party has done, or else to do the really hard work of building a new party from the ground up by getting elected in local elections, state elections, federal elections, and then and only then the presidential election. Choosing not to participate in the presidential election doesn't undermine the power of the two parties at all. It just robs you of the ability to say "no" to the candidate you dislike the most. That's really what your presidential vote is for--it's not like you're ever going to be faced with a candidate you can be completely enthusiastic about. Even Sanders had issues, although I supported him like mad in the primary.
If you like Sanders' politics, supporting what he's doing in the wake of the primary would be a good move. If you don't, get involved in the party you do like, and do things that will actually result in that party gaining power. Casting a protest vote in the presidential election is easy. Doing something that actually changes the system is hard. Please, do the hard thing. And if you dislike either of the two candidates substantially more, please consider holding your nose and voting for that candidate's opponent.