They did go through phases where stewards were instructed to try and get people to sit, but they eventually recognised it was a losing battle. They didn't bother at all last season and haven't done so far this term either. I wouldn't be surprised if there's a strategy aimed at defeating it in the new stadium on a few upstairs computers.
If they're that serious about it, they should just start ejecting persistent offenders, whoever's nearest the pitch. Just make an example, one at a time. You've no excuse if there's no-one standing in front of you, and people would soon stop if they knew they were risking getting chucked out.