As Kspalla wrote , if you are a designer you can correct for the unwanted frequencies at different stages. But if you are converting an existing system to some other band you have only duplexer and additional filters to play with.
how much rejection you need ... say you have a spuious at 2300 MHz which is above limit by 6 dB. you may need a band pass filter which passes ur desired frequency band and has a rejection of atleast 9db (including an additional 3dB margin) at 2300 MHz. If you can find the source of spurious and reduce there itself it will be the best method .