You will need to actually call SSA.
When you’re entitled to Medicare because you signed up for Part A at age 65 or later but have not yet applied for Social Security retirement benefits, you can withdraw your application for Part A. (To do so, contact the Social Security Administration at 1-***-***-****.) There are no penalties or repercussions and you are free to reapply for Part A at any future date.
If you’re eligible for Medicare but have not filed an application for either Social Security retirement benefits or Medicare, you don't need to do anything.
As long as your employer has 20 or more employees, you have the right to postpone applying for Social Security and Medicare and can continue to contribute to your HSA — until you stop working.
It is when you have applied for, or are receiving, Social Security benefits—which automatically entitles you to Part A—you cannot continue to contribute to your HSA. In that situation, the only way you could opt out of Part A is to pay back to the government all the money you’ve received in Social Security payments, plus everything Medicare has spent on your medical claims.
DO as advised in my first paragraph and call SSA to let them know your option.