Plane or polygon inner layer?
In either case, easiest is to block connection with an object.
For planes, place a circle around the pad, so it becomes disconnected. (Planes are drawn in negative space.)
For polygons, place a Polygon Cutout Region around the pad.
This is purely graphical and will not solve netlist errors.
The preferred method is to remove the pad from the one net, and add it to the other net.
Don't forget to repour polygons (T, G, A) if you don't have auto-repour turned on (still a good habit even if you do), or rebuild planes if they're in use.
If the design is pathological (e.g., inner layer poured with a Region), edit the object's vertices to add a hole around the pin in question, or just delete the fucking thing and place a new polygon in the same outline. This is, uh, hopefully, very unlikely to have been done by a human, but sometimes, imported files create such horrors as this, and you should clean them up.
Tim