Well, Gravitons are a real theoretical particle, it would be massless with a spin-2 nature, and some bosons do have anti-particle equivalents, for example W- and W+ bosons, so a Anti-Graviton, as far as i'm aware, is feasible, if a bit of a stretch.
Short of replacing it with Quantum locking (aka: Bioshock magic stationary balloons), but that would require magnets on the ground.
So gravitons seem the best option unless you want to go down the 'lol aliens did it' route.