I'm allergic to Christmas trees and similar, makes my skin red, hot and itchy, I have to be quick to wash it off and the pain goes fairly quicky as long as I haven't scratched it raw already. But I now use gloves if I touch them. Could you maybe get some coats for them when they visit in a similar way and try fencing it off with short edging fence? Our local cheapy pound shop type place does little panels, about 1 1/2ft for £1 or small trellis panels that you can stretch right out to about 3ft and lay side ways for the same. Both you could just shove in the ground and remove after they've gone.
Hope their skin is healing well