Sorry. According to official church teaching, the REASON for taking something (“removing filth from a place which should be dedicated to Him”) does NOT excuse the action (taking something that does not belong to you) so instead of pleasing Him, you are offending Him by your sin of stealing which, as I just said, is not excused by the reason for doing it.
Just because YOU do not consider it to be stealing doesn’t mean that it isn’t.
If your church was running a charitable event and spent money printing up signs to advertize it and placed those signs up all over town and someone took those signs down, would you not consider that stealing? Your church paid for those signes. It gave out money that could have been used for other things in the hope of advertizing an event but because someone took those signs down, your church no longer has the money, or the signs, nor is the event advertized. THAT IS STEALING!
Taking something that does not belong to you IS stealing. The REASON for taking it does not excuse taking it accoding to church teaching.
First off, if it isn’t ANYONE’s property, then it isn’t your property either and you are not entitled to take it. By doing so, you are taking something that isn’t your’s. Secondly, some individual PAID money to advertize something with that poster. By taking it down, you have robbed that person of the opportunity to do so and that person isn’t getting what he paid for with his money. Again, theft.
That is not YOUR decision to make unless you are running the place. If they received permission to put it up, YOU have no right to take it down.
I would inform the property owner. I would not STEAL something that wasn’t mine because STEALING is a sin and sins are offensive to God and Christ’s own church teaches that the REASON for sinning does NOT excuse the sin.