How big is your tank? This quoted output on the filter is a lot higher than the actual output.
However if you really wanted to reduce flow you could simply add filter floss to a few baskets, cram it full of biomedia, but not sure why you'd want too.
My biomaster 250 is rated to 900lph and I used it on a 55l tank, flow wasn't anywhere near being overpowering. It certainly wasn't filtering nearly 20x the tank volume.