As of vanilla support for sign editing got added this starting to cause some issues and complications when it comes to limiting who can edit signs and where. At this moment there is no event being fired when UI opens, at best we can utilize PlayerInteractEvent, but this only addresses interaction with sign block which might or might not result in sign UI being opened. For example using dies or making sign glow will not open UI at first interaction, but will do it on second one, so canceling this event isn't really an option. 

Solution would be to have new event which gets fired before opening sign UI which could be canceled if needed. Tho this should only be fired when sign is being edited or have extra indication if this is sign placement UI or edit UI to simplify determination what should happen with it.