numerous attempts to post (though the status section says "you may post") are blocked with
vBulletin Message
*** Registration has been denied (anti spam), please contact the administrator: ***

subsequent mails to remain unanswered

Another attempt with a new registration ended with you have been banned / time when ban will be lifted: never

Activity log in my profile has 3 posts, i.e. no indication that my id had been misused.

How can I continue to use the forum, please?
How can I avoid such troubles in future?