As far as I can tell there's nothing to stop a team doing this if they wanted to, but I'd be intrigued to now the definitive answer. There's a fair amount of speculation that some clubs have engineered a dip in form toward the end of a season to avoid promotion.
I believe they can only refuse to admit a team at either end if they fail to meet rules set out at the start of the season, such as stadium capacity, floodlight standards, not being in admin 2 summers running (ie 13 months plus continuous), disclosure of ownership etc. When it comes to stadium standards there's also the allowance for time to complete work, eg Cardiff getting to the Championship with terracing and having so many years to convert it, and then that being extended once the new stadium was announced because converting it would have been a waste of money. I would suspect that if teams got promoted and met the rules they would be obliged to take the place. Where it's been enforced is obviously a number of occassions a club has been 'promoted' from the Conference, can't think of a relegated club not being accepted unless Boston went straight from L2 to some division below the Conference. As the decline in performance to avoid promotion conspiracy has been suggested, Scunny last year as well were set for a massive bill to upgrade Glanford Park if they stayed up. They were on Football Focus saying they wouldn't get any support from the Football authorites because they'd been too well run and they only gave financial support to clubs that needed it, so if they stayed up they'd get the full bill and in order to stay well run would have to weaken the squad for this season by selling players/reducing the wage bill to balance the books. It might well be that they're financially better off having been relegated than they'd have been if they stayed up.
