How to use IPP FIRGenBandStop method?
There are some issues with the code.
1. There seems to be a problem number of taps used for frequency. For filtering 400Hz sine wave at samplerate 48KHz I get better results if I use tapslen more than 559. 
2. ippsFIRGenBandstop apparently is implemented as band pass + band stop + band pass. By doing this IPP tends to add some ripple at the higher end (around 44,000 and above). See here:


When using this filter it is recommended to use some kind of a FIR filter design tool like TFilter: http://t-filter.appspot.com/fir/index.html
Or matlab FIR filter design toolbox.