Altium: make a specific GND pad not to connect to GND pour
Im thinking one of the following approaches could do the job:

Cutout

Simplest solution.

Block path for plane

place an object with a different net next to the pin to block the ground polygon from reaching the pad

Rule

under design - rules - plane- polygon connect style: add a rule that excludes this part. Apply the rule to ALL and InComponent('putDesignatorHere') (or: InComponent('') AND InNet('GND'). Then set the connect style to 'no connect'. This ofc would also remove the other ground connection from the plane

as mentioned by Tom L. you can also address a specified pad in the rules using HasPad('IC3-3')

Different nets

Probably the cleanest solution: Give this Ground a different net name (eg VSSA), create a net tie object and then connect GND and VSSA with help of the net tie.