Granted, there is no easy way to kill off ALL of Legacy, but I think this method would work for a majority of the ones who are heavy PvPers.
Step 1: Show up to Halc wars, grab your quick Quest token, then wait to see if Legacy shows up to fight the war. If they do, just port out and continue doing whatever else.
Step 2: If you farm Hasla, and Legacy shows up in large numbers to just wipe out the greens, sit at the Nui for a while, or port out and do something else.
Step 3: In every other world PvP event, if Legacy shows up, just port out. Seriously.
What does this do?
Think about it for a moment, that many members of Legacy get their kicks and enjoyment from PvPing in an environment where they have domination over the opponent. But if you take away all of their PvP engagements, these people are left in a constant state of wanting. They will start to yearn for those PvP events, and probably even try harder to find them, or resort to 5v5s or 1v1s. After a long enough time off this repeatedly happening, the ones who actually play the game for PvP will start to lose interest in the game and quit outright. If enough of them quit, Legacy loses all its "Victory by Majority" power.
Now, obviously this comes with a side-effect and one that is pretty negative too.
Legacy will no longer be contested for anything, and they will have more room to get what they want, and they might even begin trolling the East Faction during CR/GR, just because they are pissed off at the lack of PvP. It's a minor setback to the East, in all honesty, but it's all for the greater good of the Server. There are plenty of other things to do on the Server, so avoiding PvP where Legacy is concerned is as easy as standing at a Nui and hitting Recall.
While I agree, it might be boring not going to world PvP for a while, you'll be better off in the long run by spending that time doing more productive things, like farming auroria mobs for purses/cloaks. You'll get gear just that much faster because your time will not be spent giving Legacy their fill of daily world PvP.
The faster we can make their members leave the game due to sheer boredom, the better the balance of the Factions becomes and the better off the server will be.