Nearsight prevents the activation of "long ranged" abilities, which seems to include any ability that can input targets via the minimap. By virtue of disabling the second cast, Nearsight will interrupt Elastic Slingshot. This may be a bug, although it is thematically appropriate for an effect that removes the target's ability to see.
Knockdown and Ground only interrupt dash and blink abilities.
Crowd control applied by one's self or an ally cannot be countered in any way (neither reduced, resisted, nor removed), with the exception of self-slows.