Do not do the abs() at this stage.
Using the sampling frequency and the number of points to figure out the resolution of each bin of the fft result. https://www.researchgate.net/post/How-can-I-define-the-frequency-resolution-in-FFT-And-what-is-the-difference-on-interpreting-the-results-between-high-and-low-frequency-resolution
Now, knowing that, and knowing that the first bin is "0 Hz", you can figure out which bins in the fft cover the frequencies you want to stop. You can now zero those locations or multiply them by an fraction to diminish them; you could use a curve to diminish more towards the center of the group if you wanted. After that, take the indices of those bins, subtract 1, and apply the same curve in reverse to the bins starting from the end of the fft result. For example, you might effectively multiply the fft result by
1 1 .7 .3 .1 .6 1 1 1 [...] 1 1 1 .6 .1 .3 .7 1
Now you can ifft back.
