To run command, and send any output the program writes to stderr, to the NUL device, you would write:
command 2> NUL
> is the DOS output redirect operator.
2 is the numeric identifier for the standard error stream stderr.
For information, the other streams are:
0 = stdin (default device: keyboard)
1 = stdout (default device: console window)
2 = stderr (default device: console window)