Personally, I like to resist a little, too. It's fun! But I rarely do it, unless it's called for in a scene, because, well, being tied up is more fun.

I think the way I'd handle it is to sit down with your sub, when you're not "playing", and explain (in your best domly voice) that this sort of thing isn't acceptable to the degree it's been happening, that there are times you're going to allow it, but not always. Maybe establish a command that stops it. (Geez. The mere thought of a command like that makes me all wriggly. *fans self)

Punishments for subs, like punishments for kids, need to not be just threats and they need to not result in something the sub/kid likes (i.e. sending a kid to his room, where all his stuff is). It also shouldn't go past the limits y'all have established.

As always, communication is the key. I find it most effective to talk about issues like this when we're not playing. Curiously enough, it often results in play.