Is there a way to freeze the mob inside a spawner?
I figured it out.
/data merge block (coords) {RequiredPlayerRange:0s}
This does not work with /setblock directly (MC-80690, so either /data merge command still needs to be run after a /setblock command or MaxNearbyEntities needs to be given as well, like this:
/setblock (coords) spawner{RequiredPlayerRange:0s,MaxNearbyEntities:0}