Output power spectrum clamping over C+L band (84 channels) is demonstrated experimentally on a standard 100-km single mode backward-pumped distributed fiber Raman amplifier (B-DFRA). The clamping is realized by pump adjustment based on a simple linear relation of individual pump power versus on-off gain level and gain tilt in its spectrum. The average clamping error over all channels are less than 0.32 dB within the experimental range.