if you buy the stuff designed to stop people or dogs in stores.. yeah its weak sauce. What you need is the stuff for bears. Seriously, its nasty stuff. Something that will fend off a 600lb animal will certainly make a mess of a human. Its too bad we cant get a diluted form of mil spec CS gas in a squirt can. Its a really mild nerve agent .. makes ya loose your bowel control and barf all over but extremely effective for managing crowds or an aggressor. Blast that into someone's face and it'll take a good shower before they're right again.
Or, as said.. get a hold of a cop friend and as him to get you the good stuff and a quick class on how to properly use it.