I don't think I'd try something as permanent as chopping down the impeller. Be really tough to keep it balanced unless you happen to have a magnetic balancer.
There's a good chance it won't be too much current, it should be a wide, slow current, so not a problem for anything but weak, useless fish like goldfish, discus, angels, etc.
Otherwise, cutting a small bit of plastic and over intake and rubber banding it in place should work ok.