If they were going to bring in a twist to let him stay, they would have already done it as they did with Helen and Marc.
They may mess with the format to a frustrating degree, but this is the one thing they can't change. Once people are up for eviction and the viewing public are voting, they are legally obliged to stick to the rules as laid out from the start. If people spent money on evicting Marco and he had more votes than anyone else, ignoring the public and keeping him in would be fraud. If that came out, it would result in a huge nationwide investigation and the backlash resulting from that would not only lead to the end of Big Brother, but probably all UK TV shows involving a public vote. (In 2007, there was a scandal involving a quiz show called BrainTeaser (also on Channel 5) where it turned out that on a couple of occasions they hadn't announced a proper winner for the viewer competitions and just had the voice of a member of the production team claiming to be a winner. As a result, there is hardly anything that has a phone-in viewer competition anymore - nearly every game show featuring one cancelled that element, just in case anything similar ever went wrong.)
The BB production team may be misguided at times, but not even they are as stupid as that.