As above, if you want flat response with rolled-off highs, the Low and High Cut EQ block should do the job. Or the high cut control in the Parametric EQ block, or global EQ, which behave the same. If the roll-off isn't steep enough for you, you could try the high cut in the IR block instead - you can use this "empty" without an IR file loaded.
Low/high block = green
IR block = blue
Both set to 6 kHz