]
The only way to prevent those bitcoins from being stolen and re-added to the supply would be to lock/destroy them in a fork. For example, we could implement a fork where coins are destroyed if they aren't moved in n blocks -- a number of years down the road maybe. This would be extremely controversial.
I'm sure it would be controversial, but I'm not so sure as to the degree. From what I can see, Bitcoin is still dominated by people who want to make money first and foremost, so I could see a move like this, which preserves their coins' value, garner a lot of support. There will be some pushback among purists, but how many really are there? Either way, I agree that the community should start drawing up some concrete solutions.
I'm personally in the camp who would prefer that lost coins be left where they are whether they're vulnerable or not, but I won't really mind if they get destroyed either, considering the scale of damage their theft may cause.