Sometimes the graph screen may get a little cluttered, but you don't want to clear it and redraw most of the graphs just to get a couple of them off the screen, and you don't want to delete the offending equations completely.
In this case, just select the equation you no longer want to see and choose Hide Graph from the Edit menu. The graph will disappear from the screen immediately, but its equation will remain in the same place in the queue so you can refer to it later.
Shortcut: Right-clicking on a curve will bring up a popup menu with Hide Graph in it as well.