double click on your instrument to get to the instrument settings. Each instrument needs a volume envelope. 5 4 3 2 1 would decay to a volume of 1 and hold that volume. If you want silence, end your envelope with a zero.
Also, there is a "key off" button. You can insert a key off by pushing the ~ key (next to the 1 on your keyboard). That is used to silence a channel.
