force OP? what do you mean by that? are they giving you links to click or is it just straight up joining the server and force opping themselves without you doing anything? (which, to my knowledge, is pretty impossible since Minehut is pretty secure). so, either you gave the wrong person OP or someone somehow got into your account on the Minehut console. think about who you've told your login information to and who you've opped, perhaps someone is not trustworthy here?
regardless, I suggest that YOU IP-ban them from your server, it might not hold them off forever but it certainly will give you peace for a little while, if you've already done this and they still have come back then I'm truly stumped. wait it out I guess, change server name or set server to unlisted for a while until they eventually forget your servers name and all about it. also, backup your world frequently so their griefing will simply be an annoyance and not a loss of huge amounts of work.