I would personally go with a Galaxy wide nerf of all markets.
Credits are far too easy to come by now.
Result: The cost of everything is devalued (it's now possible to get one of the big ships within a couple of hours of play) so everything becomes worthles
Knock-on result: With everything now being worthless, value tends zero. Gankers now do not have to think about funding their ganking and attack other commanders without any real repercussions to their gameplay.
Nerf the lot.
Make a missions pay much lower values so you have to earn you way in the game.
Make the time spent mining actually only make a lot of money if you spend a lot of time doing it.
Make Exploration only pay for newly found systems - and in those cases, pay a lot
Credits are far too easy to come by now.
Result: The cost of everything is devalued (it's now possible to get one of the big ships within a couple of hours of play) so everything becomes worthles
Knock-on result: With everything now being worthless, value tends zero. Gankers now do not have to think about funding their ganking and attack other commanders without any real repercussions to their gameplay.
Nerf the lot.
Make a missions pay much lower values so you have to earn you way in the game.
Make the time spent mining actually only make a lot of money if you spend a lot of time doing it.
Make Exploration only pay for newly found systems - and in those cases, pay a lot