How could I put an Enable pin on a standard two-stage CMOS op-amp?
To add power down functionality to the op-amp above you need to cut the current flow. You can do this by adding transistors acting as pull-up, pull-down and switches. These are typically controlled with digital signals and it could be good to buffer the control signals using the local supply voltages to reduce supply and cross-talk noise. Pull-ups and pull-downs can often be near minimum size (unless slow timing causes excess leakage or short-circuit currents that are seen as problematic), while switches may need a lower on-resistance.

To cut the current in the op-amp you have shown, you can do this:


Pull up the gate of M7 high using a PMOS
Pull down Vbias3 and/or Vbias4 
Do one of the following:


Add a switch (NMOS or transmission gate depending on the voltage level) to the input of the Vbias input you are pulling down. 
Cut the power in the circuit that is generating the Vbias you are pulling down (if you only have one such circuit it might be best to cut power in both).



For the complete LDO you also need to handle the other branches that might leak. The most important thing to do is to : 


Pull up (or down) the LDO output transistor so it turns off.