The use of shapekeys sounds like it might be a more likely possibility than the armature, if you made the changes to a shapekey instead of the base mesh, the basis shape will still be available to return the mesh to it's old shape. Note that in this case just deleting the shape key will remove the altered shape that you have done. This question may help in that case.
If you created shape keys and setup drivers (the purple values have drivers) to allow the armature to control them they can also deform the mesh. The drivers can be removed by right clicking on one of the purple values and selecting delete driver.
The armature modifier is the link that allows the armature to deform the mesh. By deleting the armature modifier you remove any effect it can have. You can also try disabling it first by disabling the eye icon.
The armature modifier uses the objects vertex groups to know what vertices each bone moves, by removing all the vertex groups you will remove any trace of the previous armature and will need to redo any weight painting.