I understand you cannot simply play an Agent to 'block' a spot from an opponent.
But, if there is nowhere else to go, do you HAVE to perform the Action on the spot?
Example: Reassigning Agents from Harbor - only place left to assign is Builders Hall. Do you have to place your Agent there AND be forced to Build?
Well you could play an agent to block a spot from an opponent, you just must be able to carry out the action, example pay for a building. Not sure if you should waste both an agent and turn doing this.
The rules are clear you must play an agent if you can, so, if there is only the Harbor spot, you must to that. You may only pass when there is no option for you then you are out for the rest of the round. If everyone is in the same boat the round ends early.
Don't forget to read the Official FAQ
Ah, somewhat misread you meaning about blocking an opponent.