Anyway, before you change the exe make sure you made a backup of it.
I assume Iztok is being a little tongue in cheek here because hex editing the executable is not really a very good idea.
I assume that he makes this suggestion because the bottom line is that there really is no way to do this. This is a hard coded function of the game that is not moddable.
So you have act's suggestion which is basically to try and make enough money so that you don't care that you're losing some percentage of your income each turn.
You have Iztok's suggestion of hex editing the executable.
And finally you have the suggestion that you don't want to hear which is to spend the money each turn.
I know it's not a good set of choices but what I and I'm sure everyone else has done is to spend the money each turn and do your best to find relatively efficient and hopefully painless ways to do this.
I know that none of these are particularly good answers but I know of no others.
Generally one reasonable thing to do is to have a bunch of ships and use your extra money to upgrade them to your highest level of technology. Upgrading is less expensive than rush buying ships so that a more effective way of spending your cash.
However the easiest way is probably to rush buy a top line ship every turn or so. If you have a number of these in production then you have some flexibility as to how much you need to spend.
Anyway 20K buys a pretty nice ship.
The bottom line is what else are you going to do with money anyway. They don't give you a refund in dollars at the end of the game for money you don't spend. The only good thing excess money does is to rush buy social projects and/or rush buy ships. If you don't want to do either then you may as well waste it and lose 20% to graft, it's not the end of the world.