No.  This object has no objective right to exist or to be possessed by anyone.  No one has a right to possess it and therefore has no ownership rights to it.  Unless of course you believe in Religious Liberty.  According to Vatican II, they do have a right to own it, and this would be theft.  This is a clear example of the ramifications of Religious Liberty.  But in Catholic reality, no, this is not theft, since they have no rights in justice to own it.

Of course not. It's not a sin to walk by and leave such things alone. I think the point is that it's not a sin when someone takes action to remove noxious filth that has no rights. For example, what if it was an explicitly x-rated picture displayed in public? The common good trumps the self-proclaimed-but-really-non-existent-rights of the filthbearers, no?

Only SCOTUS, only rogue usurped authority - may it all come tumbling down - gives to evil "rights."

Prudence dictates in each situation, and that includes assuming the risk of retaliation. God sees everything and the devil sees a lot. He will retaliate if God permits.

But your advice above was good. Let DL take it to the confessional, where hopefully he will receive correct information. 

First amendment never included satanism and hate vulgar speech.  As Catholics, the bumper sticker means hate for Jesus and Christianity.

Yes. Can you imagine if someone had an image offensive to Moslems? The Moslem would have been supported in taking it.

But when a Christian objects to something which offends the One True God, this upside-down society wants to protect the offender and do damage to the Godly man.

This is a form of White Martyrdom.