It can always be found at the bottom of the Properties Panel. For elements with a lot of properties, you will need to scroll within the properties panel to reach the element visibility section. 

To turn off an element for a specific breakpoint click on the icon and you will see a slash appear. Click again to bring it back. 

Responsive works from big to small. This means that changes on Desktop, will affect all three smaller breakpoints, but not the other way around.

When a new element is brought in on a smaller breakpoint, it gets added to that specific breakpoint and all smaller breakpoints. The larger will not show in element visibility. 

Elements can only be deleted from the breakpoint they were created on: the largest icon shown under Element Visibility.

When clicking delete on your keyboard for a smaller breakpoint it will not actually delete the element, it will only turn it off for that view.

For example, with the image below - in order to delete that element it would need to be deleted from the tablet view; I know this because tablet is the largest icon I see in Element Visibility. 

If the largest breakpoint is turned off, you can delete the element by turning it back on for the largest view then deleting it from there.