If a creature has already been legally declared as a blocker, Immobilizer Eldrazi’s ability won’t change or undo that block.

You compare the power and toughness as you declare blockers, not as Immobilizer Eldrazi’s ability resolves. For example, if you control a 3/3 creature as the ability resolves, then later that turn it gets +0/+1 such that it’s 3/4 as blockers are declared, it won’t be able to block.