Indeed, very good point.
From my understanding there is no way to prevent it in the proposal but it could be mitigated by checking the actual SPS holding of each voting account at vote closure time.
It would prevent this hack as the SPS transferred would account only for the 2nd account and not the first one anymore. It would also remove that strange need to "update" your vote.
Still that's a lot of work for this feature, it would be nice to assign a priority level to a proposal when voting just so we are sure it won't impact the development of core game features.
Easiest option is to simply "power down" your voted SPS each time you instant withdraw.
Indeed, that works as well but the important point is that at the end of the voting time, each voter is accounted for it's true amount of stacked SPS.
But yeah, 4% or 5% now is still way too much money for swinging a vote, and it will further increase is the SPS value increases as well.