First check whether or not there is a lintel above the window. If there is, as long as it's not showing signs of cracks, the window can be safely removed.If it is timber framed, remove any hinged openings,and then simply cut through the timber frame with a saw.a crowbar and hammer would be handy at this point as sometimes considerable force may be needed to ease the frame out.you can then either replace or simply brick up the opening.If on the other hand it is a pvc window, the stays holding the glass will have to be removed from the inside.then once again simply cut through frame,only this time using a hacksaw.