How to get squirrels out of inaccessible attic before plugging their entry hole?
To supplement the other answers that recommend a one-way pest exit door based on the comments that the current hole size and location will not accommodate such a door: You can always patch the current hole, create a new one for the door (preferably in an inconspicuous location if possible), and then either remove and patch up the new hole after you are sure all pests are gone or leave it in place should pests find a new entryway in the future.