When people have nothing to risk they tend to be far more reckless. All you can do is try to properly range what kinds of hands they are going all in with and then take advantage of that range. If they are pushing with any two big cards then you can take advantage by waiting for a moment when you have a Blackjack 21 because you almost always have them dominated in that spot. If they are shoving any pair, then you have to wait for a pair of 9's or better and call or be willing to flip with any two big cards. I normally just sit by and watch all of the players shove, then take note of what the shovers and callers holdings are because it will allow you to make a better judgement of their range in the future. Notes, notes, and more notes is my advice.