What do I do if my plants are growing too tall after flowering?

Regardless of strain, the growth or stretch after switching to 12/12 will continue for about 1/3rd the entire length of the flowering cycle. So if they finish in 8 weeks, you have at least 2 and a half weeks of vertical growth to contend with after flowering.

Tie the top shoots down now! You don't have any other option if the plants are too tall for your space and you're already flowering... it's too late to trim them back and if you can't raise the light or lower the plants you have to tie them back.

Get some good staking points either on the plant containers or in the tray and pull down the tallest shoots until they are at a managable canopy level. They will revert back to top growth and be pointing up a few hours after tying them down. You won't get the big huge top colas but you will likely increase your yield.