I haven't done this personally but it looks simple enough to do.N
Navigate to your Hexxit Folder
(If you're on Windows this'll likely be under "C:\Users\[Your Name]\AppData\Roaming\.technic\modpacks\hexxit"
Go to config > extrabiomes > extrabiomes.cfg
Midway down the list there's "B:redwoodforest.enablegeneration=true" I would think that setting that value to false would be your fix.
[Edit]
Found in their wiki, confirms the above.