moving line objects

It can be difficult to grab the object you want on a crowded slide, especially if the object is not filled. One technique you might try is locking the objects that are inadvertently being selected, to prevent them from being picked up.

I'm not sure what you mean by "mark" the objects. If an object is locked, even if it is selected, it will not move when you move some other object. You can, for example, selected a locked object along with an unlocked one, and the cursor keys will only move the unlocked item. To make an unlocked shape easier to move, you can even select it along with locked items, and then fill the shape -- all the locked shapes will remain unchanged. Once filled, it is easy to grab the unlocked shape with the cursor, move it, and then unfill it again.

Gernot, when an object is unfilled, the only way to grab it is by the line (the stroked area).