Hi pakoman,
You just need to create a custom crt_range, limiting the horizontal frequency range to whatever you want:
monitor custom
crt_range0 15625-16100, 49.50-65.00, 2.000, 4.700, 8.000, 0.064, 0.160, 1.056, 0, 0, 192, 288, 448, 576
Here I've set the highest value to 16.1 kHz.