Hi Involute, you can delete the circle without deleting the cylinder. Probably what's happening is the cylinder was selected since you just dragged it. If you right then click on the circle to select it, both the circle and the cylinder will be selected because MoI uses multiple selection without needing to hold the shift key down. If you want to select just the circle after you have dragged the cylinder you will need to click in empty space or hit the Esc key first to deselect the cylinder before you select the circle.

If you drag the circle around immediately after the extrude, you'll see the cylinder move around too - that's the history update mechanism which is a link between the original curve and the cylinder. You can turn that off by selecting the cylinder and using Edit > History > "Disable update" button but it isn't necessary to do that before deleting, deleting won't make a history update.

- Michael