that's impossible. All you can do is talk to them and tell them it's a bad idea. The data will exist from now until the end of the current human age. Everyone wants it, 23 and me may be decent custodians, but eventually it will end up in the hands of every government, corporation, and entity interested in getting DNA information through breaches and some eventual buyer of the company selling it.
I remember a Simpsons joke where Homer finds out the government has everyone's DNA on file and asks about it and they say "Yep everyone who's touched a penny since 1932" or something.
Turns out it didn't need to be that elaborate, you could just ask folks to mail it in ;)
That is exactly what I did. And I got a temporary mailbox at the local UPS Store so they could not get my real address. And signup on their website was with a new email address. It’s a pain but it was worth it for me. Of course, I opted out of the DNA Relatives feature.
My sister didn't have an account when we sent the packages. When the data got analyzed it showed it as "My Sister" with close to 100% accuracy. The packages were anon, so they couldn't know which sent which.