To stop the playback of a sample/loop phrase, press and hold the ENTER button and then press the EXIT button. All currently-playing sounds will stop.（ALL SOUND OFF）
Note: The decay of any effects, the playback of a Song, and/or the Click will not stop when using this method.
You can also assign the "ALL SOUND OFF" function to a pad connected to TRIGGER IN jack AUX3, or to a digitally-connected pad that's assigned to AUX3, and even use use a footswitch (e.g. BOSS FS-6, FS-5U) to stop the sound.
Refer to “Assigning Functions to Footswitches or Pads (CONTROL) ” in the TD-27 Reference Manual (PDF).