Not voting would have been the only answer, as the other parties are basically controlled opposition. Yet, not voting would also count as a unpatriotic move, thus exposing you? I don't know. Sure enough, Russia has a story of antidemocratic centralised powers, be they Tzars or party leaders. And for any neighboring country, that has always been a problem.
invalidating the ballot. Not not voting. There is a big difference. I am not sure how the terms are in English and whether there is a parallel, but you can "vote against everyone" - your count will count to the total percentage vs your vote is lost.
And yes, some people were absolutely forced to vote. But make no mistake, even if the elections were "free", the actual result does not matter at all. Numbers were drawn no matter what happened during the election.
What actually matters and I am very disappointed to hardly see any coverage in the media - a ton of people showed up to Noon Against Putin. Even in Russia. Navalny's last wish was for people to go vote on the last day at noon and the lines were impressive. (It is not a meeting, you are literally just standing in line to vote, they can't arrest you.) If you're unhappy with the current state of affairs, just show up. Even if you have already voted, just show up on the voting point at noon. And people did. And they saw that there are many.
Needless to say, with few places you were able to vote abroad, and less persecution to be feared, the lines were humongous. Surely, the Berlin embassy made sure how to cause a disgraceful bottleneck to keep people from voting before it closed.
Btw, you can see exit polls from Russians outside of Russia made by independent volunteers on voteabroad.info . But keep the bottlenecks in mind and that many people stood in line for 9+ hours and didn't get to vote, so they are not represented in the results.