easy way to remove all past events

another way is to backup first your wp_em_events table then use some queries to delete past events using phpmyadmin

e.g.

delete from wp_em_events  where  event_start_date between '2011-01-01' and '2012-12-31'

do I only need to delete these from wp_em_events? no additional data stored anywhere else?

just saw that every event is also matched to a post – so I’d at least have to delete these posts as well, no?

yes, you can also delete entries in wp_posts; maybe you can used join and using wp_em_events post_id