How do I make a simple Redstone Door with a lever that villagers can't go through?

The most simple door that a villager cannot go through is an iron door powered by a button. If you need to use levers, you will need to build a simple XOR Gate.

Buttons

You will need:

• 1 Iron Door
• 2 Buttons

Put down the iron door and place a button on an adjacent block to the door on both sides. When you press the button the door will open momentarily, with just enough time to walk through.

Levers (XOR Gate)

An XOR Gate is relatively simple to make, and allows you to open the door using levers on both sides of the door. I am using XOR Gate Design D from the wiki.

You will need:

• 1 Iron Door
• 3 Redstone Torches
• 4 Redstone Dust
• 2 Levers

The XOR gate is useful for controlling a mechanism from multiple locations. When controls (such as levers) are combined in an XOR gate, toggling any control will toggle the XOR gate's output.

In our case, the output the XOR Gate is toggling is the iron door.

I chose this design because it is compact and simple (doesn't involve repeaters or pistons), but there are other XOR Gate options to choose from if this one doesn't suit your needs.

With both of these methods you can open and close the door by flipping the lever or pressing the button. Villagers cannot get through becuase they cant interact with buttons or levers.

Quote from the "Door" page on the wiki:

Since villagers cannot operate buttons or levers and zombies cannot break them, iron doors can be used to lock a house in a generated village.

you can use the commands /replace and /fill to make a neat door. just connect a button to a command block that puts an huge iron wall open/close by making an XOR gate with a command block on one side that fills the gate and another that replaces it with air. It is very simple if you follow the steps carefully.