WordWeb Online

1. Made impossible
   "Obviating the effects of heat and cold on time-pieces"
   - preclusive

1. Do away with
   - rid of, eliminate
    
2. Prevent the occurrence of; prevent from happening
   - debar, forefend, forfend, deflect, avert, head off, stave off, fend off, avoid, ward off

See also: obviation, preventative, preventive

Type of: forbid, foreclose, forestall, preclude, prevent

Antonym: necessitate