After trying various ways to stop spam such as captchas, abuse filters, blacklists and whitelists. Some of which were intrusive, while others slowed the wiki down. I've now had success with a mini Extension:ConfirmAccount. Setting this up is explained at https://www.mediawiki.org/wiki/Extension:ConfirmAccount#Minimal_settings
Now instead any potential editor has to be approved manually, it only takes a moment if an admin is watching the site. That is they request an account by confirming their email address, providing a user name and a short message to prove they're not a bot.
Rob