One of the bitternesses of the Flemish towards the Walloons is that the flemish policemen are more watchful than the walloon police so they have to pay much more traffic fines. Maybe a quick way to convince walloons to become independentists would be to switch the polices between the two communities: Flemish police in Wallonia, walloon police in Flanders...
They could also offer to switch the tax comptrollers, in this case I suspect that the opinions would switch, the flemish would ask to stay belgians and the walloons would ask for independence...

Ironically, this has been happing already for many years, to some degree at least. Not paying taxes is something of a national sport here, but it hasn't hurt Belgium all that much.

As long as we get half of the flemish willing to get along with Belgium, we, Belgians, have enough to continue. It takes less energy to keep things going as they are than to change them....

Lycurgus: Perhaps the best way to kill the Belgian state would be for the Flemish people en masse to ignore it, to cease paying taxes, to stop voting, to stop utilising whatever facilities still remain within its remit...The Flemish do not need to declare their formal independence at this stage all they need to do is to stop the flow of monies to Wallonia and to treat the Belgian government with the contempt and scorn that it merits...
 
Who will mobilize each and every Fleming not to pay the taxes? What year would this be effective? What if income-tax evasion charges are issued by the courts?
 
If every Fleming or at least the vast majority of them are mobilized and organized, such civil disobedience can work. Otherwise, it is doomed to fail, with the few resisters facing prosecution by the state.