Delete or disable keyframes

To delete any number of keyframes, select them, and then press the Delete key.
To delete one keyframe in the Graph Editor, Ctrl-click (Windows) or Command-click (Mac OS) a keyframe with the Selection tool.
To delete all keyframes for one layer property, click the stopwatch button to the left of the name of the layer property to deactivate it.
When you click the stopwatch button to deactivate it, keyframes for that property are permanently removed and the value of that property becomes the value at the current time. You cannot restore deleted keyframes by clicking the stopwatch button again. Deleting all keyframes does not delete or disable expressions.
To temporarily disable keyframes for a property, add an expression that sets the property to a constant value. For example, you can add this very simple expression to the Opacity property to set it to 100%: 100.
Click the Enable Expression button to toggle the expression on and off, which toggles the keyframes off and on as a side effect.

If you accidentally delete keyframes, choose Edit > Undo.

Jeff Almasol provides a script on his redefinery website that automatically removes keyframes based on specified criteria—for example, all keyframes in the work area, all odd-numbered keyframes.