I like what Judith Thomson has said about these questions. She argues that we should distinguish between 'infringing' a right and 'violating' it. In her terms, we infringe a right when it is not wrong for us to do X even though someone has a right that we not do X. There may be some overriding moral concern that justifies our infringing a right. (For example, assume that the only way for us to get a child to the hospital and save her life is to cross someone's property; but the owner has put up "no trespassing" signs. In this case, we may infringe his property rights.) These cases are interesting for a lot of reasons. For one thing, we may owe something to the person whose right was infringed. (See p. 135 in Reading Nozick.)

Perhaps a right can be infringed because of an overriding moral good. Perhaps because of another right that is more 'stringent.' Thomson sees property rights as less stringent than the right not to be killed, for example.

In any case, Thomson argues that rights are not "absolute" (p. 135, Reading Nozick). Some can be infringed, and she tries to say when we can do so. For example, she suggests that a property right can be infringed when the property at issue is of "little value" to the owner, but not when it is of "immense value" to the owner. See "Ruminations" p. 139-40 in Reading Nozick.

The bottom line is this: libertarian property rights are too absolute to fit our everyday moral convictions on property. They need to be limited and balanced against other moral concerns.