Hi Charly - this is what is called the "seam" of a closed surface.

A NURBS surface is kind of like a sheet of rubber paper. If you want to make a piece of paper into a tube/cylinder, there will be a line where two of the edges of the sheet come together and touch each other. This is the line that you're seeing.

You can't delete it because it is a part of the surface. But if it is getting in your way and you don't want to see it, you can select it (click on it a second time after clicking on the object once), and hide it.

Eventually there will probably be some global way to turn off the display of all seam edges.

- Michael