This is a really good idea. No need to build anything, and its easily removable as well.This must be very annoying. This is 100% on the owner but many times we have to deal with the consequences of other people not taking responsibility.
This may not be feasible for you but, you could get a motion sensing sprayer that attaches to a water hose. Goats hate getting wet and a good spray to the face should discourage any more visits.
Maybe I'll suggest some payback for the lilacs would be some goat meat this fall.
