The auto-complete suggestions for a particular field contain other values for that label (in your example “Email”). Currently, there isn’t a preference to ignore certain values.
That being said, I have a couple of suggestions which may help accomplish what is desired:
If you no longer have a use for your old job’s email, remove every instance of it within Codebook. After it’s removed, it should no longer suggest it.
If you’d still like to use your old job’s email (or at least keep it in Codebook), but don’t want it to show up in the auto-complete suggestions for the “Email” field, you could create a new label (let’s call it “Email-Old”) and change each instance of the old job’s email to be associated with the new “Email-Old” label, this way it won’t show up in the auto-complete suggestion when you create a new “Email” field. You can add a new label under File -> Customize Labels
When using either of these methods, the search function in the upper right hand corner may be useful in locating all instances of your old job’s email.
Let me know if either of these solutions work for you.