Leave him alone, and certainly not dump cold water on him.
Having lived in SF awhile back, I can also testify that many of the homeless are frighteningly aggressive, and can get quite menacing (as mentally ill and drug-addicted people do) when attempting to intimidate old ladies into giving them cash.
The "water sprinkler" question is almost moot, since they have now all been removed: but what now? Forcible removal? Cops at the doors every day?
Nobody wants a "solution" involving barbed wire, dogs or handcuffs. So seriously, as a caring person: what do you say?