you can get grommets from any motor factors. maybe a few pence each. (i think Halfords do them but theyre expensive).
to remove your wiper you will first need to undo the nut on the spline of the wiper arm. approx 10mm. there is usually a palstic cover over the spline.
then go inside your boot and on the inside you should see a long black trim piece held on by approx 5-6 plastic black round clips. carefully remove the clips by prising them out (beware they brake easily)
then inside your boot you will find the mechanism and motor. these are held on again only by 3-4 nuts (if i remember correctly). undo these and then pull the spline through the hole.
use your grommet to fill the hole.
as an afterthought i would go into the engine bay and remove the rear feed from the washer pump and plug the hole on the washer pump with a bolt etc or you could put a screw/bolt into the the rear feed in the boot where youve just removed the washer nozzle.