You cant remove the post, you can only hide the particular post from your own view or highlight the persons name and click the mute button. This is the "other" side of being on a decentralized platform that we have to deal with. There's virtually no sensoring of comments. You can flag a comment but there's no authority figure or anyway (that I know of atleast) to actually remove the comment or even ban the individual from your page. Best thing to do is to simply put him on mute if he's being an asshat.