It's been a hot second since I've been active on this lovely site. A long while ago I heard about being able to lock a knuffle so that it can still level up, but it wont change its appearance (Or something similar).
Is that an actual thing we can do now? Or do I just need to avoid feeding/playing with the knuffle in question?
You can lock one to a set appearance at level 100, you are given an option to choose the form it stays in permanently at that point, but it's a one shot thing, once you've selected a form, you can't change it again.
In order to do that once it's hit level 100 you only have to hit Fix State at the bottom of it's page and it will give you the choices then.