If you don't use the trem on your Strat, you might not want to leave it sitting there, all wobbly; worrying you about tuning and flutter and whatnot. You could shove in all five springs and snug the trem-claw up as tightly as possible to hold the bridge hard back against the body. That'll do the trick.
Or you could do a Clapton on it and 'block' the trem.
This just means shoving in one or more blocks of wood to fill the cavity around the sustain-block on the bottom of the bridge. Sometimes, you can get away with one piece, where the bridge sustain-block already butts up against the front cavity wall (like here). Other times, you'll need a block in front and behind.
It's important that these blocks are a snug fit, and preferably of a nice hard wood. The holes on this block are (I think) an old, Dan Erlewine (the king of guitar repair knowledge) trick—since the block's good and tight, if you need to remove it, you can insert a screw to give you a grip. Clever.