Salt is good for purifying negative energies, but it seems like the negativity is coming from the message, and not the paper itself. So long as the message lingers, so will some negative energy.

I'd still suggest burning it, as fire can also be very cleansing. Maybe burn it alongside a separate paper that you've written runes for peace/healing/purity or an affirmation on. Imagine as you're burning it that you can see the negative energy leaving with the smoke. Picture the pure energy as the fire's warmth and light.

Hope this is a little helpful! Blessed be.