Make it indifferent to a specific person or group blowing it up, not to anyone doing so.
Then you have to somehow define a specific person (you don't want it to not count the future iterations of them) and blowing it up (you don't want it to figure that they blew it up in the last case, on the basis of the butterfly effect. Both of these ideas are easy for a person to understand, but don't seem to mean a whole lot on deeper levels.
I just noticed that LessWrong has not yet linked to FHI researcher Stuart Amstrong's brief technical report, Utility Indifference (2010). It opens: