Well, since you already have a hole through the body for the tremolo and a bridge, your best bet is to simply block the bridge. Get a couple of chunks of wood, and simply put them in the gaps in the trem cavity on the back of the guitar between the trem and the body. You really can't replace the wood that was removed for the hole, so there is no where to bolt the hard tail bridge.
Another option is to really tighten the springs on the back so the bridge won't move at all if it is too lose for you already.
i recently bought another new strat myself, and i dont really use the trems either. the problem with those floating bridges (i guess you know this) is that when bending notes while playing the bridge moves foreward and sometimes the strings dont go completly back in tune. so like someone suggested, just buy a couple more springs (2 more is fine) to put in the back.
you'll probably have to unscrew the 2 screws that hold the spring anchor enough to add the new springs. this will keep the bridge from moving foreward and messing with your tuning. you can do that and/or add wood blocks on both sides of the bridge in the bottom/back of guitar. its about the simplist way of making your guitar play like a hardtail.
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