If we are getting spammed on a daily basis, we can turn on moderation of posts of new members.
That way posts of newly joined members will have to be approved by an administrator before they will be visible for on the board.After a new member has posted one or two approved messages, moderation can be turned off.
This is a slight inconvenience but if it will be explained as an anti-spam measure in the welcome message for new members, I don't think it will meet much resistance