it's not about "keep away". players can still play to score goals, but having them play 1-touch, or scoring only from headers, or something along those lines will definitely slow the rate at which goals are scored. that isn't something that you start at 9 - 0 either...more like when you get to 3-0 in the first 10 minutes!
Keeping a low goal diff isn't always possible. But if you can't get creative enough to keep it at least "reasonable", then you aren't thinking hard enough.
I don't think that there is a magic number, but some common courtesy and being a good sport is not hard to figure out.
