The Subscribers feature allows capturing email addresses of users interested by your updates, directly from your Timeline. If you are using another solution for this purpose or simply don't have a need for this feature, you can disable it as follows:
Open a project on your projects dashboard.
Select "Timeline" in the menu bar:

Scroll down and uncheck the option "Collect email addresses":

Click "Save Changes". That's all, enjoy!
Open a project on your projects dashboard.
Select "Timeline" in the menu bar:

Scroll down and uncheck the option "Collect email addresses":

Click "Save Changes". That's all, enjoy!