Why not just remove credits from the game full stop and put it as hours played to unlock ship's etc and make it a maximum of 1 hour play a week, that will stop the whiners complaining about people earning money using the games mechanics??
Seriously I have not used any of these things for making credits other than I did a few hours of passenger missions just to see what could actually be earned but I didn't do it enough to earn the billions I know some did.
I really have no issue with people having more credits than me because it doesn't affect my game in the slightest.
I only had a problem wit these missions because they were massively out of whack and a slap in the face to anyone doing massacre missions. You can get 5 million credits in one mission for killing 8 skimmers vs. a similar massacre mission that will involve you going into a CZ (dangerous) and killing 30 - 108 highly armed ships?! And on top of that, you can quickly dispatch the skimmers with dumbfire missiles from a ship instead of using an SRV?! Really?!
Heck, even doing that in an SRV is still silly - skimmers aren't that deadly and the (potential) loss of an SRV is cheap.
And then on top of that you can stack multiple missions, not even have to go to the other target sites (as long as the faction is the same) and just sit there, firing missiles, and cash in.
I've never complained about any of the other popular cash farms - Robigo, Sothis/Ceos, Smeaton runs - because they all involved some element of danger and/or took some time to complete. But these skimmer missions are just crazy. You should have to kill way more than 8 for that kind of payout - or deal with serious base defenses.