1. Bring up the highest members of every tribe to a table and discuss different roles with them, a council could work. Whoever chooses not to come to the table becomes a pariah (fully use the media to make it clear that it was their choice to not think on a national level as you take away their rights). No need to kill anyone, just don't consider them when enacting public policy and every time they complain be sure to explain that it was a choice, their choice. Publicly.I don't think you know how destructive tribalism is in Africa. they are extremely territorial and distrusting of outsiders. How will you even go about bringing proposals to the table with people who view those who live only a few hundred kilometers away as the enemy. You can't get them to trust and work with their fellow countrymen, goodluck getting them to open up to other Africans
2. Prop up every tribe's positives, make that a grand spectacle if you must. Advertisements of every ethnicity/tribe on TV showcasing their identity. Force media to follow that script. Shrink their "freedom of press" in this regard.
3. When citizens choose to make irrational decisions due to tribalism make penalties inhumanely high. Be brutal in this approach. Show that there's a price to pay for non-tolerant behavior that can in any way shape or form harm the social or economic fabric of the country.
Start all of this off by reminding every citizen that "There is no time for bad blood..." and go from there. People like protagonism, so every member of every tribe that was invited to come to the table will come...and those that don't, you work their image to show the people they represent that maybe these people shouldn't be representing them in the first place.
Play on cultural pride, heavily.