I mean physically knock her out.

I'd say just about any decent street-leveller could do it.
 
The thing to consider is since her powers are psionic, even if she's using them unconsciously, it is her mental endurance and not her physical endurance that matters. Although she has superhuman physical durability her mental endurance is still that of a 12-14 year old girl which has been shown to have strong limitations. On certain occasions Nico has given her a can of soda to pep her up when she becomes fatigued during combat. A skilled and disciplined fighter with greater endurance than her (wouldn't have to be superhuman) and enough agility to dodge her punches could take her down. The degree she uses her powers to is directly proportional to how fast she is worn out, so the stronger the punches shes dishing our and taking, the faster she'll go down. The ideal candidate would be Captain America, since he could absorb those punches with his shield.
 
She does have a pretty good track record of tagging enemies though. I can remember her landing haymakers on Punisher and Wolverine. Whether she unconsciously uses her psionics to predict where they will be or if they just weren't expecting a little girl to smack them around is a reasonable point of contention. I'm inclined to believe the latter considering their facial expressions post-suckerpunch.