Stuns, Silence, Knockups, Knockbacks, Suppressions, Polymorph, Taunts, and abilities which interrupt channels all interrupt Katarina's ultimate.
If this is about your Amumu game, than you should know that Amumu's ultimate isn't a stun. His ultimate is an entangle, which prevents movement and autoattacks. Katarina's ultimate is a channeled ability and the entangle crowd-control "does not interrupt the channeling of abilities. " (from the wiki)
In that Amumu game against Katarina, the abilities which could disrupt her ultimate were:
-Amumu Q
-Riven W
-Riven third Q
-Diana E
-Morgana ultimate (only if the tether lasts the full duration will it stun)