Off-topic is an appropriate place to post, and I'm glad you did not include details, as the rules state that all public areas of the site are to be kept K-rated.
To answer your question, report him to the mods (but don't expect much of a response) for sexual harassment, then block him. Whatever you do, do not respond to him further... that just encourages those people.
I'm sorry that happened to you. I know several of our authors have been approached by others for 'requests' or 'co-authoring' (which usually means write their story for them). It's not unusual, but that he would go into suggestive detail is a new one.
~~Wiccan~~