Koh Samui mayor Ramnet Jaikwang declared today that the island is imposing a 3 week entry ban for the Island, effective until the end of April 2020.
Bangkok Airways (who also own the airport at Koh Samui) also announced on their website that they are closing all three of their airport (they also own Trat Airport and Sukhothai airport) until the end of April. In addition to that they also announced that they are suspending all domestic flights until the end of the month.
The Island will not be completely shut off from the rest of the world though, there will still be ferry service available for those wishing to enter or leaving the island, but entries are subject to health certificates and quarantine upon arrival.