Thank you for the feedback. All blocksets are indeed deletable and editable, provided that the timer has not hit zero. When the timer has hit zero, all settings that can be used to "cheat" are disabled. Then basically all you can do is add sites to blacklist and remove sites from whitelist.

Edit:
In version 1.4.1 there is a new setting called "Settings Protection" with which you can edit the behaviour of disabling settings. Also now you can unlock settings by doing a typing test, which makes "cheating" just a little bit more annoying, but still allows changing settings when necessary.