I was also thinking about something similar to an oil slick.
Since the vehicles hover, however, oil wouldn't work.
If the vehicles are made of a metal that is effected by magnets, then a magnetic puddle of some sort could work....

Usually oil slicks either make you lose control slightly, or spin a little....

The magnetic puddle could have chunks of magnet latch on to your vehicle and make your steering be over effective.(you try to steer a little, but it makes you steer a lot)

Ah, I know!
How 'bout a fuel air bomb like the grenade in the game Resistance.

Or, a mine of sorts that when flown near, shoots up from its burrowed spot in the ground, exploding like a hedgehog grenade (also from Resistance).

Maybe a Decoy Projection weapon/support system, which projects slightly flickering copies of your craft in the nearby area.

A gravity grenade would be cool...
I saw some python scripting related to something similar the other day, so it shouldn't be too hard.

example of use: you drop the grenade, then the vehicles behind you are drawn towards it (semi-magnetically)
It could be slightly disorienting, or just slow them down a little.

I like the magnet idea. Maybe you could have different levels of magnetic force that you can unlock.
I was thinking it could be fired into the barriers at the side which would pull the other player that went past it of course and maybe even crash them with a strong one.

I think I have seen a mini-gun on some of the vehicles, that looks REALLY cool! Maybe there could be a shield unlock that could stop bullets but not any force field type thing so magnets would be unstoppable.

EMP. disables nearby enemy engine and weapon systems until it wears off