A nonconforming use that is intentionally discontinued or changed to a conforming use for a continuous period of 18 months or more shall not be reestablished, and the use of the structure or site thereafter shall be in conformity with the regulations for the district in which it is located; provided, that this section shall not apply to nonconforming dwelling units. Abandonment or voluntary discontinuance shall include cessation of a use regardless of intent to resume the use.