The neighbor is never allowed to enter your property and destroy signs. The proper thing would be to handle the matter in court. Regulations vary from homeowners association, city, county, and state for signage, however visibility at points off of the property will be key to establishing liability for damage.
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If you can prove that the neighbor entered onto the property or destroyed your signs, that could be grounds either for you to press criminal trespassing charges or to file an anti-harassment petition with the court. An anti-harassment order (like a restraining order) would order the person not to have any contact with you or your property. If the neighbor ignored the order, s/he could be arrested for the violation.