NoAdsMixi.css

/* NoAdsMixi */

#adBanner,
.adBanner,
.diaryBottomAd,
#footprintjack,
#footprintjack02,
.messageAreaAd,
#oldContents table tr td iframe,    /* for list_music.pl.  ... */
.prContents,
#prSepecial	/* mixi's typo. :-p */ {
    display:none ! important ;
}

/* SimpleMixi */

#globalNavigation ul li {
    display: none ;
}

#globalNavigation ul.utilityNavigation {
   float: right ;
}

#globalNavigation ul.contentsNavigation li.home,
#globalNavigation ul.utilityNavigation li.logout {
    display: block ;
}

#attention,
.checkMobile,
.greenpower,
#headerArea h1,
#keywordRanking {
    display:none ! important ;
}

広告を消すだけなら、

iframe {
    display:none ! important ;
}

だけで十分だったかも?
でも、広告以外にも消えるものがあるかもしれんので明示的に。