Hi! Thanks for reporting this!
This was actually just overlooked from my side. Web removal was implemented as a "reaction" to player's direct action, so when a Spider faints due to spell effect, it is not triggered. This is a bug, and will be fixed in the next update!
In the meanwhile, if you're unable to continue the game, please use the "Erase saved game" button in game's options. Sorry for the inconvenience!