You can specify security mode when calling the Connect() method using the SslMode parameter (see API reference).
Connect with no encryption (port 21):
ftp.Connect("test.rebex.net", SslMode.None);
Connect with explicit encryption (port 21):
ftp.Connect("test.rebex.net", SslMode.Explicit);
Connect with implicit encryption (port 990):
ftp.Connect("test.rebex.net", SslMode.Implicit);