So, if I am not mistaken 0.5 ug/ml is 0.197 mols per liter EtBr

this is an abstract from 

Ethidium bromide: destruction and decontamination of solutions. (Armour Method ( Lunn, G.and E. Sansone)

Ethidium bromide in water, TBE buffer, Mops buffer, and cesium chloride solution may be completely degraded by reaction with sodium nitrite and hypophosphorous acid. Only non-mutagenic reaction mixtures were produced. Destruction was greater than 99.8% in all cases; the limit of detection was 0.5 micrograms ethidium bromide per milliliter of solution. Ethidium bromide also may be removed completely from the above solutions by using Amberlite XAD-16 resin. The limit of detection was 0.05 micrograms ethidium bromide per milliliter of solution (0.27 micrograms/ml when cesium chloride solution was used).

I haven't tested this next method but maybe follow this reaction at slight molar excess of the neutralizing reagents.