Date: December 28, 2010 04:46PM
Baptism is supposedly for the forgiveness of sin. But there can be no forgiveness, according to Mormon theology, for denying the Holy Ghost. That is the "unpardonable sin." See the scriptural references in the LDS Topical Guide "Holy Ghost; unpardonable sin against"
So, unless your Mormon relatives are willing to deny the truth of their own scriptures, you can head off post-mortem baptism by denying the HG, and notify them in writing that you have done so. Many people have done that - see http://www.blasphemychallenge.com
and search Youtube for "blasphemy challenge".
Point out to them that they are putting their own souls in jeopardy if they submit your name for (re)baptism, since by doing so they would be denying God.
It's worth a try.