Short of screwing it to the face of the guitar, you can't make it technically fixed--but you can make it virtually fixed.
Loosen the strings so you've got some slack to work with. (Or do this when the strings are off during a re-string.)
Take the rectangular cover off the back of the guitar. From there, you'll see the metal block that is the bottom of the tremolo. The trem block is attached to a "claw" with springs (there's spaces for 5 springs, but there's likely going to be 3). The claw is then screwed to the inside of the body. But the claw isn't pulled snug with the body--this distance is what lets your tremolo float.
Again, with lots of slack in the strings (or no strings), simply turn the claws' screws so they pull the claw towards the body and away from the tremolo, this will pull the tremolo closer and closer to "flat." Turn the screws all the way so that the claw is snug up against the wood of the guitar, and back up on top, the trem will be "locked" flat against the body.