This page ought to show how to close a connection as well!

Doug Lerner 12 years ago updated by Alexander Blach (Developer) 12 years ago 1
When you connect to an (S)FTP server, Textastic will use up to three simultaneous connections to speed up file transfers of many files and folders. You can configure this amount in the connection settings screen (setting "Connections"). You can edit a connection's settings by tapping on the blue arrow button on the right side of each connection.

Since Version 3.1 all connections are automatically closed after a file transfer is complete.