I guess if the all we have to worry about is some fool scratching his declaration of love onto the stones then we're on to a winner. What I mean is if this is the biggest threat that the stones face then all is well. Of course this is not the case elsewhere but in the case of Castlerigg, it's a robust circle made of large tough stones that have survived thousands of years of Cumbrian weather, ploughing, animals, trippers, new age fools climbing all over them, and even christian purges. So I guess at least at Castlerigg we can say that the stones are fairly safe.
Considering the volume of visitors to Castlerigg it's hardly suprising that some damage may occur. Take the long view, Castlerigg is robust enough to act as a Cumbrian freak magnet thus keeping the dozens of surrounding Cumbrian sites relatively free of nutters.
One thing that I would advise is to beware of knee jerk reactions to one-off incidents. Do you really want to see Castlerigg caged like Stonehenge.