TinyTask is supposed to watch for the key presses Break or ScrollLock or Pause (the emergency stop keys), and immediately stop playback. Another method is the "Playback Hotkey" combo that should terminate a playback, which is "Ctrl + Shift + Alt + P" by default (note the 4 keys all held down). This key combo can be changed in the menu: Prefs > "Playback hotkey".

If it still doesn't stop, it is possible that another app has captured the keyboard or otherwise interfered. Fortunately, future versions of TinyTask (being developed in 2019) will include the ability to pick any key or combo you wish. (Note: the Escape key is not a good choice for emergency stops, since it's so commonly used for normal interactions during recordings.)