Altium: Designing a switch with 3 pads, 2 positions but only single contact
First: You want a Single Pole Single Throw switch (SPST).  The switch in the Altium library is likely Single Pole Dual Throw (SPDT).  Simply delete one of the T-pins from the PCB footprint. 

You need to edit the switch PCB footprint, select the unwanted pin, delete, and save. You can copy the whole library with the switch in it, or just create a Altium PCB file and place the switch.  Then use the Design-> Create library from PCB file.  Then you just edit the switch model and delete the unused pin.