Unfortunately, once Biden wins the nom, I expect a lot of Bernie Bros to sulk their way into either not voting or spite-voting for Trump.
Speaking from the perspective of someone who could be considered a "bernie bros" equivalent in UK politics, yes refusing to vote for a candidate I didn't believe in is what I would do.
However, we have more options in the UK as to who to vote for in other parties, so its not as if my vote would be withheld completely.
I am sick and tired of the politics of fear of "must beat this person at all costs" rather than encouraging people to vote for meaningful change. The next UK election will be "beat the Tories at all costs" with Labour as a the front runners but, if I don't like their policies, they won't get my vote.
The main political parties thrive on that sort of politics, relying on that for victory, rather than offering anything palpable.