Here's a quick review of the Automation modes:
WRITE: Write mode overwrites any existing automation data. If you have several nodes in an envelope, and begin playback in Write mode, the envelope will be erased for everywhere within the played-back region and overwritten with the current or new parameter values.
LATCH: Latch mode honors existing automation data UNTIL you make a change to a parameter. At that point, the envelope will remain latched at the new value, overwriting other keyframes or changes in the played-back region.
TOUCH: Touch mode is similar to Latch mode in that it honors existing automation data until you make a parameter change. However, once you change a parameter value, touch mode automation will then begin to adjust the envelope slowly back towards the NEXT keyframe value. For instance, if you have a keyframe at 03 seconds and another at 15 seconds and you begin playback in Touch mode, if you make an adjustment at 5 seconds then release the control, the envelope will slowly return to the 15 second parameter value over the next 10 seconds.
To alter a section of a track, use the appropriate mode for what you are trying to do. If you want to overwrite everything in the region, use Write. If you want to overwrite SOME of the automation, use Latch. Use Touch if you want only want to modify some aspects of the envelope, but not necessarily everything.
To clear an automation lane of all keyframes and reset it to its default value, there is an eraser button in the automation lane panel on the left. This will clear all edit points. To remove a single edit point, click and drag it outside of the automation lane (up or down) and it will "snap" away.