When you have one Jezz in an area by itself, keep closing it in until it literally has no space to move. It will freeze, and you will have one less Jezz to deal with and some space cleared. I call this "the still point", and it is very useful.
Two walls can be built at one time, a red and a blue one. On the higher levels, it sometimes makes sense to start building walls closer to the edge, so if a Jezz crashes into it, you'll have at least one half to build from. Also, you can start building other walls once one side is completed. Careful, though- if the two walls collide, they'll be stuck there, nothing you can do, just bait for the Jezzes.
Clear a line down the center so that one Jezz is by itself in one half and the rest are in the other. Whittle down the side with more than one Jezz (or on level 1, either side) as far as you can get it, preferably to the still point, and then pare the sides of the other half (vertically) until you reach a percentage near 74%. Then rush in, wait for the Jezz to get to the very bottom and clip off all the top half you can manage! I've gotten 94-95% of the grid cleared using this strategy, although it gets dangerous to use this method on the higher levels.