I think a change of format is in order for the last ever series of BB. To stop the bigger characters leaving first. Either switch to vote to SAVE every week or use my slightly more complicated idea below
2 vote lines for each contestant, one to save, one to evict. People can chose this way to either kick out who they dont want, or save their favourite. This way the people who cause arguements etc are more likely to stay, thus making the program a bit more fun to watch.
Take the negative votes, away from the positive votes...and bang, theres your evictee, the one with the lowest total score. They would probably make more money that way too...