James, please provide us the steps how we can reproduce this issue on our Demo.

I did this;

1. Signed out of Geek.Zone so that I am a guest

2. Hovered over a post

3. Clicked flag

Please go here: http://geek.zone/admin/dev-tools/languages

1. You'll see "add new text" button to the left of the search, click on it

2. in the popup window, choose "Base" as the section

3. for the key field type sing_in_to_flag

4. for the Text in English field type "You need to sign in to flag content

5. Click add

Wouldn't it be better to turn off flagging for guests in the first place? How does one do that?

Yes, that's seems like a redundant feature, you're right, I'll report that to our developers and it will be removed in one of the upcoming updates. In the meantime follow the instructions above. 

Topic was moved from General Questions.

I can see that it would be useful to allow guests to flag in some situations, but it would need to actually work. The option to turn it on or off would be better than removing it entirely.

No, I meant it will be removed for guests, it will still function for logged in users