I wouldn't normally begrudge them taking a food item but the item in question was an Eternal Steak from the Artifacts mod. It's a food item that normally isn't consumed when eaten, so it can be re-eaten over and over. Would there be a way to add items to a blacklist? I could do the config with the item ID myself.

There doesn't appear to be a blacklist in the configs for food items seagulls can't steal, but you can disable seagull stealing outright.