Everyone is correct.
Do NOT let them come to your house and tell them to stop the harassment as of now or you will have your legal counsel take over.
This is ridiculous.
I would also let them know that you will be in contact with Petsmart if they do not cease their harassment. Petsmart needs to be told about their practices.
They will not take that to the next level. They already know they have no legal right to do so, and they are facing loosing their location (Petsmart) to conduct their rescue business, which they will NOT want to happen.
Petsmart lets them use the facility, but they will not continue to do so if their customers are being harassed in that fashion, esp. with no legal rights to do so.
Just tell them you were seeking out legal advice and were told to cease all further communication and interaction, and will NOT allow the home visit because they already said that they will make this visit a deciding factor on whether they might (unlawfully) seize the cat.