The real answer is that there's really nothing you can do about it as an infantry. Kicks and shield bashes don't really do anything to people constantly holding down their block, and shields have a ridiculous amount of HP. It's one of the reasons why I can't stand playing infantry in this game. You don't have the ability to really do anything against a turtle on your own. The combat is very unsatisfying.
In Mordhau, people that just block lose stamina faster than the people attacking them, eventually leading to a defensive player that doesn't attack back getting disarmed and dismembered by his opponent. Kicks against people that had their shields up would also get stunned just long enough for them to get hit once, which wasn't the case against people without a shield. Kicks in Mordhau could be used more offensively than kicks in warband.
In For Honor, there's guard breaking and bashing to make extreme defensive play more difficult. The defensive player has the options to counter the guard break, and dodge out of the way of bashes, but pulling it off is a lot harder than just holding RMB.
In Warband, if you were just constantly holding down your block, you just opened yourself up to getting spammed down and having your shield break really fast. Kicks were also actually useful, but those were more for punishing face huggers rather than turtles.
In this game there's literally nothing to punish a turtle, other than them having their S key movement speed slowed, and making it easier for them to get surrounded by your teammates. Requiring the help of your teammates just to take down one very defensive player that is just holding down RMB with a shield and S keying is very unsatisfying and boring gameplay.
