How to disable SpeedStep for an Intel Core i7 processor?

This will vary from board to board, but there should be an explicit BIOS option for "Intel SpeedStep" or "EIST" that you can disable.

EDIT: Also, I believe you have it backwards. SpeedStep is responsible for downthrottling, not upthrottling; you'll run at the base clockspeed if you turn it off. Turbo Boost is what upthrottles (but I believe SpeedStep does have to be on for that to work).

DOUBLEEDIT: You shouldn't need to, but you can also try disabling C1E support.