Ivan,
The BB60C spur reject option will generally remove input-related spurious (signals generated from image frequencies, harmonic mixing products, etc.). What it cannot remove is residual signals from internal clock multiples. It also cannot remove intermodulation products from the input itself (e.g. strong 101 MHz and 103 MHz produce low-level 99 and 105 MHz IM3).
If you see the signal with no input, it is probably a residual clock multiple (typically 10 MHz intervals, sometimes stronger 80 MHz multiples), which unfortunately cannot be removed. These should be below -106 dBm at maximum sensitivity, and are typically around -120 dBm except for a couple of spots.
If you suspect intermodulation products, increase the reference level. This will decrease artifacts from intermodulation.