You can't really block acrylic.
If all you want is to even out the edges and make things a little easier to seam, I'd pin the item to the correct size, then either spritz with a water bottle or cover with a damp towel, and let dry.
Just like acrylic won't felt, it also doesn't "set" stitches well. If your yarn is a blend, you may have better luck than 100% acrylic. Once your item is put together you could also try a trip through the wash to 'even out' the stitches a bit.
Hope this helps!