If a station is silent for 12-consecutive months, the FCC will atomatically cancel the license, and in the case of an AM license, the frequency is deleted from the community of license. This is not an FCC rule, but rather a CFR (Code of Federal Regulation) rule mandated by Congress. Additionally, if a licensee intends to take a station silent, an STA (Special Temporary Authorization) application is required to be filed and approved. If a licensee intends to remain silent more than 12-consecutive months, said licensee must file another application within the 12-month period specifying the licensee intends to return the station to the air. The licensee may then take the station silent once again within 24-hours of returning to the air and remain silent for another 12-consecutive months. This is legal and will not cause the licensee to lose said license. However, it may prevent the licensee from renewing said license at license renewal time as the FCC may view the continuous silence as not being in the public interest.
As for fines, I am not aware of any licensee that has willfully refused to pay a fine. However, many licensees will appeal a fine, in which case the fine is not due and payble until such time that an Administrative Law Judge has made a final ruling in favor of the FCC or in favor of the licensee. In some cases the ALJ will rule the fine is justifiable but may reduce the fine amount due to any number of factors including a hardship for the licensee.