Uhm, what would then happen to the bus at the end of the line? The bus has to go somewhere - no real bus line just ends.

In most cases you can just go back to the first stop with the line if you just want a line from A to B. Ofc it's not super efficient to have the bus go the entire length of the route with no stops.
Btw you can select which lines to display by using the info view button on the left. Click on busses and it'll show all your lines and the eye symbol lets you hide the ones you don't need to look at. Makes editing individual lines much easier.