Break Contract

You target a creature that has signed a pact/contract with a an otherworldly creature such as a devil or celestial (But not limited too). You render that contract null and void, ending its status. 
Any effects of the contract such as ability score improvement and spells are now removed. Warlock classes are unaffected by this.
The breaking of the contract my have unforeseen consequences such as drawing the ire or the contractor who may wish to exact revenge on the either the one who broke the contract or the one who originally signed the contract. It their viewpoint, they may have violated terms. 
It is up to the DM to handle the fallout.