There's a couple ways to do it. The best solution is to thread a machined screw hole into the block of your trem right in the middle between the bridge posts (might not be in the middle of the block since some blocks are offset). This should probably coencide with the spot where the middle spring is inserted in the block. Then I would use a fairly thick and short allen screw with a wide head. You then just screw the allen screw in the block so the head of the screw is looking up the neck, and the threads are poking out pointing to the bottom of the guitar. Then it's a matter of shortening the screw so it doesn't interfere with the trem being lowered, adding some super glue to the threads, and calling it done.
You would also want to tighten your spring claw quite a bit. If you have a blocked trem and the tension on the spring claw is set as if the block weren't even there, it can cause rattling and vibration problems. It will also cause the trem to drop a bit when you do bends (like it does without a block). It also allows you to detune without bringing the other strings out of pitch. All that happens when you tune the low string down is that extra tension lost by the string gets transferred into the body through the blocking screw. Since your bridge is blocked from moving, it won't bring the others out of pitch.
Or you can get something like
this that screws inside of your trem cavity under and parallel to the springs. You would screw the chrome screw into the body, and then extend the black allen screw till it touches your trem block and keeps it where it needs to be.
I would NOT do
this. This is messy and keeps you from using the trem entirely. This can't go up OR down with wood on both sides of the trem.
On a couple of Ibanezes, I used strips of hardwood that were JB welded to the block and were sanded/filed to the right size. I super glued some felt tabs on the wood where it hit the body to keep it from making noise when I'd drop the trem and reset it. This also worked fairly well and was completely reversible. All I'd have to do is knock the wood off the trem and chip off all the JB and I'd be back to stock. But the fact that it was wood and was held on by glue made me uneasy.
The best solutions I've seen, however, are provided by some guitar manufacturers. Both the kinds of guitars I play have ready made trem blocks built in.
First
my ESP horizon. It has a lisenced Floyd style bridge, but they all came with an extra fine tuner machined into the block so you can keep your trem from moving backwards. I extend it to touch the body and I tighten the crap out of the spring claw. Even when I bend the stiffest strings, it won't go out of tune. I never use the trem, but if I wanted to, I could dive bomb all day.
My Parker flys also come with a trem blocking system. Their trem setups aren't floyd like at all. They use torsion bars instead of springs, they have a spring tension knob at the bottom of the guitar, and there is a lever on the back of the guitar that moves a block into and out of place. The block is supposed to be there so you can set the "neutral" position of your bridge easily, even if you want a floating trem. You just block it, tighten your spring, tune up, and then loosen your spring until it starts to come out of tune. You then flip the switch to unblock it and then bring it back into a full tune using the spring tension wheel on the guitar, and bam... you're perfectly in tune and your bridge is perfect. No tools, no bullshit.
Of course, on the Parker, I also leave it blocked and have the spring tension cranked up so it doesn't come out of tune when I bend.
