WinLockInput locks or unlocks input to message queues.

#define INCL_WIN /* Or use INCL_PM, */
#include <os2.h>

HMQ     hmq;      /*  Message-queue handle. */
BOOL    Lock;     /*  Lock indicator: */
BOOL    Success;  /*  Success indicator. */

Success = WinLockInput(hmq, Lock);