1) It's they who do it.Thanks,
2) It's them who do it.
3) It's they who does it.
4) It's them who does it.
Only one is correct and that is number 1, although it is unlikely that you would ever see it. It seems clumsy, so most English speakers would reword the whole sentence eg It is the plumbers that do it, they are the ones who do it etc.
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...it's you who likes to tease her.I want some more light shed on this please.