The proper way, would be by removing the fan completely, making good the brickwork and plaster, both inside and out, then decorating the damage.
Quick and less satisfactory way is to remove the fan, maybe squirt some expanding foam in to fill the hole, then cut/fix a bit of ply to cover it temporarily.
Remove fans then soil pipe end for now.
Proper way is to fill with polystyrene or foam to insulate then plasterboard in hole level. Outside remove damaged bricks then cover new bricks and surrounding bricks with duck tape to keep clean and cement new ones in. Pull tape after a few hours and bricks will be spotless.
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