How do i delete a certain part of my drawing in a certain viewport without deleting it in model space? (Cadd LT 2008)

Viewport is only a view to a common (single) model. So you cannot delete it from a single viewport.
 
Of course you can have some objects individually inserted only in paperspace. You can also suppress the object display (without deleting it) - there are individual layer settings (off/on) per viewport.

This is where good LAYER management comes in handy! If the objets that you want to remove from the viewport are on a layer of there own all you have to do is activate the viewport (double-click inside viewport), go to the LAYERS MANAGER and do a VP FREEZE on that layer. This will turn those objects of in the current viewport and not delete them from your model.
 
Hope this helps!