Prevent, preclude, obviate, avert, ward are comparable when they mean to hinder or stop something that may occur or, in the case of prevent and preclude , to hinder or stop someone from doing something.
Prevent usually implies the existence of something which serves as an insurmountable obstacle or an impediment.
Preclude differs from prevent in stressing the existence of some situation or condition or the taking of anticipatory measures that effectually shuts out every possibility of a thing’s occurring or of a person’s doing something.
Obviate usually implies the use of intelligence or forethought; preclude also often implies these but sometimes it suggests the operation of chance. The chief distinction between these words when anticipatory measures are implied is that obviate usually connotes an attempt to forestall disagreeable eventualities by clearing away obstacles or by disposing of difficulties.
Avert and ward, the latter usually with off, differ from the other words of this group in implying prevention of an approaching or oncoming evil. They suggest therefore immediate and effective measures in the face of what threatens.
Avert, however, suggests the use of active measures to force back the evil before it is actually encountered.
Ward, on the other hand, implies a close encounter and the use of defensive measures in order to avoid the evil or to diminish its disastrous effects.