Any antenna or tower that is not used for a continuous period of 12 months shall be considered abandoned. The owner of such antenna or tower shall remove same within 90 days of receipt of notice from the city notifying the owner of such abandonment. Failure to remove such abandoned tower shall result in declaring the structure a public nuisance. If there are two or more users of a single tower, then this section shall not become effective until all users cease using the tower.