Get people to serve as volunteer park rangers, wearing an official-looking shirt or vest and carrying a two-way radio.  The rangers'  job would be to politely deter the feeding, by explaining why it's harmful. Official-looking personal action will make a stronger statement than signs.  It will like also discourage other undesirable activity in the park, e.g., people not cleaning up after their dogs, or drunks lying on tables for a snooze.