If you can identify the beginning of the block yourself (and the opening brace is at the end of the line, as you stipulate), you can stick 0 and do { # in front of it and get the block commented out. It doesn't let you get away with syntax errors in it, but I would think that a block would have to be properly formed for block comments to work, anyway.

...
0 and do { #if (whatever) {
...
}
...
0 and do { #while (whatever) {
...
}
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