Ask HN: Can I cancel an already accepted job offer?

I would handle it diplomatically but firmly, and stress to Company A that it has nothing to do with them, and that you'd love to maintain a relationship despite the awkward position you've put them in.

Your hiring manager, the people you interviewed with (6-8 people probably?), and some of the management at company A will know you rescinded your signed offer.

What are the chances that one of those folks ends up working a company with a job you're applying for 4 or 5 years from now when your application comes through?

If you want to be a little nicer about it, say that you have a competing offer and if they don't match walk.

You don't owe them anything.