how to prevent short circuit between thermal pad and vias on it as it is recommended that exposed pad should not connect to GND
Change the designator of the vias to be the same as the pad, i.e. 25.

Then, go ahead and add a Net Label to the schematic for its pin. Altium doesn't like it when things don't have net labels.

 (In the menu bar)



Here in my case the pin is named GND, but it's currently not connected to anything.
You should no longer have short circuit violations in your PCB.