And once the damage is done, it's irreversible.

一旦造成危害，便是不可逆的。

Once someone clicks to open the file, the damage is done.

一打开这种文件，电脑就会中毒。

The damage is done. Or, we could say: the genie is out of the bottle.

已经对他人造成了伤害。或者，我们可以说：the genie is out of the bottle。

If this is about an apology, the damage is already done.

如果你想道歉 我只能说大错已铸。