Love Locks and Public Nuisance Legislation
According to Mill's harm principle, the state or society may only interfere with an individual's actions in order to prevent harm to others. This is generally taken to restrict paternalistic or moralistic legislation, which raises questions about many generally-accepted laws (including those on drugs, incest, seat belts, etc).
I wasn't previously aware of the trend for couples of signify their love by attaching a lock to a bridge and throwing away the key, but apparently this has been a 'thing' since at least 2009. Moreover, enough of these 'love locks' can actually damage bridges. As reported (see article in link before last), some places, including Rome, have banned these love locks.
Is this ban consistent with the harm principle? On first sight, perhaps not: it would seem to be stretching the notion of 'harm' to suppose that they are. Admittedly, Mill does suggest that 'public decency' might provide grounds for certain restrictions, which seems to be allowing a limited place for something other than harm to justify interference. However, on further consideration, such a ban might be upheld as a protection of property rights. I wouldn't want someone attaching a lock to my fence, even if the lock didn't in any way damage the fence. Presumably, there's little reason to object to a bridge-owner saying that locks should only be attached to her bridge with her consent.
Matters are slightly more complicated when it comes to public bridges, where there is no particular owner as such. Why shouldn't members of the public attach love locks to a bridge that is (in part) theirs? I suppose, however, there is an answer to be found based on the fact that the bridge does not belong to that couple exclusively but to all members of the public. One cannot, for instance, freely block a public road, because that interferes with others using it for their own purposes. While love locks don't prevent others from crossing a bridge, they do amount to one individual (or one couple) assuming rights over public property as if it belonged to them alone.