It's not possible to disable Smart Add overall, but you can disable its date detection; this detects dates not specified with the ^ character.

You can disable this in the Account Settings:

1. Open the Remember The Milk web app.
2. Click on the Settings gear at the top right.
3. Click on Account Settings.
4. Click in to the General section.
5. In Date Detection, click on "Off" to disable the date detection.
6. Scroll down and click Save changes.