This can't really be done without incentivizing SP splitting and other manipulative games. If my vote doesn't benefit from the effect of my vote, then I will vote first with a small account, then with the larger one. The smaller account would benefit from the effect of the larger account instead of the (now-disallowed) larger account benefiting from itself.
It helps to think of a large account as being a whole collection of imaginary small accounts (say 1 SP each) that happen to be working together. Changes that try to put caps or limits on the ability of one account to fully benefit from its SP will encourage people to make these into actual small accounts.
Even coalescing all votes from a single IP into one pool wouldn't prevent this exploit; proxy services are common enough.
yeah, whole can of worms trying to track whos got what sockpuppets.