The way I always do this for models with subgroups is:
Right click on groups and make sure Autoselect is ticked then when you click on a subgroup it will select all the faces (this should also happen when selecting a material), then simply right click in the UV Editor window, select delete, this removes the faces, then delete the subgroup as its now empty and if it had its own unshared texture delete that too in materials (just to keep things tidy and prevent loading textures not needed). When you have it how you want it just save that as a seperate new file, don't over write the original .. and make sure you have the original backed up, backups are good.