One reason cats like to rub against people's legs is because they are rubbing their scent on you and marking you as part of their territory and a member of their pride. It's a form of affection and acceptance. A lot of cats are loners, but if you have one that's always begging you for attention, take that as a positive thing. To them, you're a very large and strange looking cat and they've accepted you as one of them. That sounds strange, but it's true. If by "you can't afford to ignore them" you mean the cat in question will scratch or bite you if you ignore them, they have the dominant role. You have some work to do. Be firm and correct them for the bad behavior. You, as the owner, must establish that you're the Alpha male of the pride.