I’m not sure that feeding wild birds is such a great idea but not really a strata matter unless perhaps the cockies are screeching at unpleasant hours.
As for playing on the grass, I’d say that putting up a notice to prohibit that might be interfering with the reasonable use and enjoyment of the common property (or similar wording you probably have as a by-law). I’d quote the by-law to the EC person and point out their sign would be a breach. Unless of course there were some short-term good reason such as having just reseeded the lawn so they wanted people to stay off for a short time.