Usually I just 'hide' the parts I don't want but you can also select parts from the tree in the main window. Deselect the ones you don't want to show.
It's much easier to hide components as James said.
There is another alternative you can use instead of going through the whole tree.
Switch to model Tab and press Tab or Shift + Tab to hide and Show components of an assembly.
- To hide a component:
Move the pointer over a component and press Tab.
- To show a component:
Move the pointer over an area containing a hidden component and press Shift + Tab.
After hiding whatever you want, switch back to your simulation study.