A thing where you can only use lock-on until a certain level could work, but not if you can just reset your multiplayer stats and start abusing it again. I'd like for the "normal" gamtypes to be listed as being made for people new to FPS. That alone will encourage people to look past it to the "real" modes where there is no lock on, and make them want to move up when they're ready even before they start. And then the "hardore" modes would be listed as the normal modes.
The simplest mind trick in the world; instead of normal and hard, have easy and normal. The game instantly feels harder without being harder, and gives the player a larger sense of achievement.(this only works with even numbers, though, as it's rather fishy to have a game with super easy, easy, and normal modes.)
This I think is probably the best way as it encourages but doesn't enforce conformity, and also clearly sets the table for a competitive standard through the "normal" gametypes. If you want to play on easy and abuse lock on? It's there. And maybe make it so you level up faster on the "hardcore" modes for some extra incentive. Double exp, maybe. That way, after a certain point, there's even more encouragement to go to the "hardcore" modes as, even though quite possible, it just takes a bit longer to level up on the "easy" modes.
There are really a lot of ways you can prevent a hardcore mode from completely splitting the community, and there is no middle ground for lock-on, as as soon as it assists in aiming, it becomes a problem, and as soon as it stops, it becomes useless. --- 2% of GameFAQs users have this in their signature. If you're one of the 98% that doesn't, copy and paste this into your signature.