I know it's no easy job, as I regularly try to remove vehicles from clan stacks. At some point they just know you are there and take more care of the vehicles, but it's still doable. Convince people to man tanks is a good way to go once they take more care of their vehicles. Usually I go for a scheme like this: Start with transport + melta/rokkit combo, if they move too much/once they start moving too much switch to tank strategy. Once you're out of tanks go for las. Usually I don't like this gameplay as it's kinda cheesy, but with clanstacks, if you can manage to take out most of their vehicles games often tend to be balanced and fun.