I didn't know one could "like" as a guest. I thought one had to create a sign-in name, with a password, and then have their "account" verified by email, as I did. Things that make you go, hmmm.
Guests can only read posts, like posts, and view attachments. Among other things they can't post, view links, or start new topics. We implemented stricter registration when we started getting bombarded by spam bots. Thus far it's kept spam almost non-existent.