Refusal of registered mail

Typically you send a plain first class letter plus the certified letter. I believe the law will presume they received the letter(s) even if they refuse them.

Your CC&R's may even have a statement that it is assumed the owner receives a letter after the association mails it.

So go ahead with the fining process; that's what I would do.

I went through this same thing. What you need to do is run an ad in the local or other public newspaper in the LEGALS section or advertising pages requesting their contact for a period of time. If they do not respond then you may go ahead with whatever the matter is that is inside the registered envelope.
However, I would suggest NOT doing this on your own but having an attorney to do this legal notification. That way your HOA is protected. This "Notification" in a public newspaper is considered an appropriate legal means of trying to contact a person. Many times this is done in divorce cases where the spouse refuses to be contacted by other means such as registered mail.
After a legally determined time period (Depending on each state) this "AD notification" will be considered as good as someone opening their registered mail. In my case, our ad had to run 2 times each week for atleast 30 days. The legals section is only printed in our paper 2 times a week. We were foreclosing on an owner's home for non-payment of dues owed for almost 2 years who refused contact. If your person doesn't live locally, you may have to run an ad in their local paper to be sufficient with notification. Talk to an attorney once the matter involves foreclosure or lien because you will need them to finish all the other related paperwork.
Ps. You can also include the costs of sending the unopened registered mail and/or Advertising into your legal bill.