There is no alternative to junk mail from a consumer point of view. If the junk mail you receive is addressed to you, it may be possible to request that the company delete your information through GDPR legislation, which would prevent addressed junk mail. However, there’s no way to stop unaddressed junk mail, and it should be simply recycled.
The alternative for businesses and creators of junk mail is to simply use email for their communications instead. Once a customer has received junk mail there is no way to find a feasible alternative.