Technically, Cloud's party never does kill the Zolom. It's more appropriate to say presumably that Sephiroth killed the Zolom's mate and that shadow you see in the marsh is the remaining Zolom (not some kind of duplicate like you all seem to think implying it procreates asexually). That would mean that Sephiroth remains the only being capable of killing the Zolom. Of course, after having killed Diamond, Ultima, Ruby, Emerald, Jenova, a Bizarre Sephiroth, and Bahamut Sin, you'd think Cloud would be able to kill the Zolom at least by the end of AC.

But there are two possibilities remaining. One being that Cloud and company may have already (finally) killed the Zolom between VII and AC or after AC since we're not shown the Zolom we can't know for certain. The other possibility is that the Zolom has continued to grow more powerful over the years and not even Cloud's crew can defeat it meaning that if they really want to get rid of the remaining Zolom, they'd have to purposefully resurrect the only known Zolom-slayer, Sephiroth.