How to remove connected passwords on Wi-Fi on Mac
EDIT 2 Sorry, misread the question.

Your System Preference pane is probably locked. The screenshot you posted is from the "Advanced" section. 

Go back to basic network preferences, and look at the bottom left corner of the window. There should be a lock saying "Click the lock to make changes".
Click the lock and enter an administrator username and password.

Go back to the "Advanced" settings and the controls will no longer be locked.

EDIT 2: Un-check "remember networks this computer has joined" to prevent your Mac from remembering the networks.

In Network preferences, remove the network whose password you want to enter each time.

In Keychain Access, search for the name of the network whose password you want your Mac to "forget". Then right click on it and click the item that says Delete "item name" where item name is the name of the password item.

EDIT 1

If you want to prevent panes from locking, go to "Security and Privacy" preferences, unlock the pane (this one will always be locked) and click "Advanced..." in the bottom right. Un-tick "Require and administrator password to access system-wide preferences".