Below, you can find a couple of examples of slogans that you might like. You can use them and change them however you want, it's entirely up to you :) However, I believe that it would be best for your own development if you utilize them more as inspiration than a ready made answer for your assignment.
Don't take that money, it's not all that funny!
[directed at people taking bribes]
There's no such offer, one cannot decline
[one always have chance to say no to a bribe]
Do not turn away, when you see a dirty pay
[nobody should be idle when they see a bribery]
On law you should rely, not a shady aid
[in the long run, corruption makes a country weaker]
You shoudn't have it, if you hadn't earned it
[if you haven't worked honestly, you shouldn't be paid]
No sweat? No GET!
[just like the preceeding one]
I really do hope that you found my answer helpful :)