When Katarina activates death lotus, can people like Morgana or Ryze or Maokai stop her channel?
Short answer: Morgana can stop a Katarina with the stun off of her ultimate but not her snare. Ryze cannot stop a channeled ability. Maokai can use his knock up to stop channeled abilities.
Snares and roots are considered movement modifiers. They cause the target to "halt movement entirely, whilst still allowing auto attacks and abilities that do not require movements." Source (For further reference here is a full list of all the CC types and their effects).
Further information about channeling spells:
All channeled abilities can be interrupted by enemy champions using any "interrupt" crowd control effect on the channeling champion. Interrupts do not include slows, blinds, or snares. They can also be interrupted by the player by giving orders such as walking away or activating another ability. Using most summoner spells will not interrupt channeled abilities. Source