You can open the cps file in a text editor. In the Fusion 360 Post Process dialog there is a button “open config” next to the post processor selection. If you click that it pops up the cps file in a text editor. Use “File->Save As…” to save a copy into your local save folder with a different name. You want it in a different folder because the one it opened from will get clobbered every time Fusion 360 updates. I think for Windows this is: C:\Users<yourusernamehere>\AppData\Roaming\Autodesk\Fusion 360 CAM\Posts
More on this in this thread: https://forums.autodesk.com/t5/hsm-post-processor-forum/where-to-put-cps-files-for-fusion-360/td-p/6093729
If the G28 is the thing you want to get rid of, I think the line you’re looking for in the cps is:
writeBlock(gFormat.format(28), gAbsIncModal.format(91), words); // retract
If you comment that out you won’t get any of the G28’s which might not be what you want since it is intended to be the retract. You may instead want to code in your own.
That line generates different things depending on the axes passed in (that’s the “words” array there at the end). In the sample I’ve got here, it gives a G28 G91 Z0 twice and a G28 G91 X0 Y0 once.