I've seen "FIRE JP" signs at the Rogers Centre during the 2000's haha. Don't recall anyone ever getting kicked out. We've seen signs calling for firings of coaches, GM's, Team President's etc. As long as the signs aren't vulgar/offensive, and the person isn't disrupting the game by holding the sign up during innings and obstructing views of other fans around. You should be able to hold up a "Fire Ross Atkins" sign at the Rogers Centre.