The only possible course of action I can imagine, if he doesn't resign and can't be sacked by the House through any particular constitutional channels, is for someone to do the semi-unthinkable and actually try to turf him out at Glasgow North East in the next General Election. The SNP opposed him in 2001 and 2005, even if the Tories and Lib Dems didn't.

It's an extremely long shot, and it doesn't solve the immediate problem, but it's the only vaguely constitutional approach I can think of.