Does an RL agent still learn if its actions are "blocked"? Say we have a game that is a maze environment where there is a character to be controlled through the maze. When the agent (the character) approaches a wall, it may try to execute an action that would lead it into the wall, which is not permitted. Instead of executing this action, we place an if statement in the code that checks if the action is permitted, and if not, does not execute the action, and simply proceeds to the next state.

Another similar example is if an RL agent is being trained in a stock trading environment, where say it tries to sell more stock than it actually owns, or buy less than the minimum amount. Similarly as before, we place an if statement that checks for these conditions and either allows the action (allows the trade) or moves on to the next state.

Does the agent still learn, even if we "override" it and block certain actions? How else would we go about restricting the agent to certain actions in certain states e.g. if a wall is to the left of the agent in the game environment, how would we restrict the agent to only move forward, backward or right, but not left?
By overriding the Agent's action, Agent can theoretically take this action over and over and do nothing else, there is nothing to stop the agent from doing this, or teach the agent that this is not the intended action here.

What I usually do in these situations is, I punish the agent if the action is not desirable. So if agent take the action to go to the left, but there is a wall on the left side, I would not change the state of the environment (So the agent won't move), but I would also send a minus value (punishment) as a reward. This way after some training, the agent would learn that this action is not desirable.  

The same can be applied to you Stock example. So if the agent tries to sell more stock that it actually has, you just don't sell the stock but also punish it with a big minus reward. This way it makes it easier for the agent to actually understand the environment.

Hope this helps.