If you want to set a user as inactive, a quick way to do this is:
From your dashboard select Learners followed by Overview:
Search for the user in question and select the More icon. This will reveal a drop-down menu which includes Make User Inactive.
At the next screen, enter a Leaving Date (even if they are not leaving, and you just want to temporarily pause their ability to log in) and then make the User Inactive.
And it’s done: