Pretty sure you'd use CUserCmd:RemoveKey( IN_DUCK ) inside a SetupMove hook
https://wiki.facepunch.com/gmod/GM:SetupMove
https://wiki.facepunch.com/gmod/CUserCmd:RemoveKey
Garry's Mod Wiki
SetupMove is called before the engine process movements. This allows us to override the players movement.
See Game Movement for an explanation on the move system.
Garry's Mod Wiki
Removes a key bit from the current key bitflag.
For movement you will want to use CUserCmd:SetForwardMove, CUserCmd:SetUpMove and CUserCmd:SetSideMove.