Legal basis
Consent, and a real reason to write back
Processing of contact form data rests on the consent you give when submitting the form, and on the association's legitimate interest in answering correspondence. Server log processing rests on the legitimate interest in operating a secure website.
Consent can be withdrawn at any time with effect for the future — a short line to the address above is enough, and no reason has to be given. Withdrawing consent does not affect the lawfulness of processing that already took place.