You can have simulated anti-tank weapons.
Much like airsoft is about people being honorable and calling their hits, you could use the same system for a tank.
Make a prop bazooka or something and put a laser pointer inside.
Use a NERF gun.
Allow people to carry "C4" (if they touch the tank, it's destroyed)
Make it so that in order to destroy the tank you have to hit key points, not just anywhere. It'll make it a little more challenging to destroy that way.
There are many ways to make this work, you just need to be a little creative.