"I get nothing from it but the pleasure of providing a place for like-minded hobbyists to hang out and compare notes. I never asked anyone from Europe to sign up anyway."
Subject to the usual IANAL disclaimer, I would say you are over-thinking this. Unless you have misappropriated email addresses to add users without their knowledge & consent (unlikely) then European members have already elected to "opt-in" to your BB by explicitly signing up to join. As such, there is no special requirement to re-request subscription or suchlike as you already have the user's consent to the existing relationship.
Henceforth, provided you take all practical measures to:
1. secure European users personal data from unauthorised access or misappropriation (including on-selling details to third parties)
2. provide a clear way for European users to request access to such information as you hold on them and,
3. provide a clear way for European users to unsubscribe from your BB and be assured that all their personal data is expunged
you will be complying with the spirit of the GDPR. In your case this could be simply catered for by a prominent sticky giving a way for users to contact you with their requests if you haven't already incorporated this.
To be honest, anyone worth their salt handling personal information in such circumstances should be applying these principles no matter the user's location.