Hi, welcome to GoY, really there isn't much you can do, except pinch out the growing tip, which will prevent them getting any taller, but whereas you would normally pinch out any side shoots, {I presume you're growing cordons} I would let1 or 2 to develop lower down, so that they can produce tomatoes as well, you would normally expect them to produce round about 6 trusses before stopping them, but with the height restriction, this is the only way I can think of to give you more tomatoes, Derek.

Commercially, they gently turn the growing tips around, and send them back down the supports, reversing again when they near the ground.

I was thinking along the same lines as Tug. You can drive in a huge 4 foot stake with lateral slats about every 8 inches or so. That way you can zigzag up then zigzag down.