My question involves a power of attorney in the state of: OR

About 12 years ago, my 2 brother and I each inherited a piece of commercial property from our parents. I own the property and the deed has my name on it. About 8 years ago, I gave my parents power of attorney to handle matters in regards to that property. Now, I want to revoke the POA. I sent a notarized, certified letter revoking the POA. They've refused to accept the certified mail. My questions are, 1) Even though they've refued the mail, does the revoking letter I sent have legal affect? 2) If not, how else, or what other means do I use to get them to accept the letter? 3) Even though my parents have refused the mail, if I sent copy of the revoked POA to the current tenants, and a letter requesting that from this point, they need to deal directly with me, and not parents, does this have legal affect and do the tenants have to as the letter directs? 4) If I were to send the letter to the current tenants, can I address as Dear current tenants or to whom it may concern, or do I need to know their names, in specific? Since my parents have not shared or shown me any tenant contract or other information, I do not have this information.

Your quickest reply will be appreciated.