I'd rule that as tough titty Player A.
It's everyone's responsibility to make sure that the line of fire is clear when you take your shot re other strings coming into view when being reset at another target.
Otherwise it would be another excuse for people to take another shot .... " Ooooh did you see that ? I hit the string from that other target ... I'll have another go " .
If a string from another target has got caught on a branch or something and is now stuck across the kill zone of your target ... then call the marshall BEFORE you take your shot and get it cleared.
Just my opinion on that one.