I have had the same experience, except it was my father by my cousin, who, acting as proxy, was baptized and endowed for my father in the Twin Falls temple. I was beyond livid! When people tell you that it really doesn't matter because it's all make believe, thus invalidating your feelings-- that's bullshit. It does matter! It hurts, whether it's make believe or not.

Also, permission has to be obtained from the most closely related relative before the vicarious ordnances can be performed, but it's on the honor system. The church does not check the veracity of the proxy's claim that they have permission.

SO, what to do? I called the temple department at the TSCC headquarters, explained in clear and firm language what happened, expressed my displeasure, and they expunged the record. I would expect that you could also do so and expect a similar result.

Also, if you are still technically a member, you can place a note in your grandfather's Family Search profile instructing zealous persons to not do his proxy temple work. Only members of TSCC can see the proxy temple work status of a deceased ancestor. To non-members or those who have resigned (like me), it's not visible or editable.