1. (在信的结尾)写结束语;(挂电话、广播节目结束时)就说到这，到此为止
If someone signs off, they write a final message at the end of a letter or they say a final message at the end of a telephone conversation. You can say that people such as entertainers sign off when they finish a broadcast.

2. 注销(领取失业救济金的)登记
When someone who has been unemployed signs off, they officially inform the authorities that they have found a job, so that they no longer receive money from the government.

1. cease broadcasting
get off the air
as of radio stations