I don't run into this problem commuting but I see it on organized rides where you would expect it anyway. I don't like having people locked on my rear wheel unless I know how they ride AND I feel like working together since drafting takes a lot of head space. Here's my take:

1) If someone caught you, you should let them pass at the pace they used to catch you. You already blew your chance to stay ahead.

2) If they rode too hard to catch you and actually aren't that fast, drop them and resume your normal pace after they can't draft

3) If they are strong enough to keep up but will neither pass nor give you a reasonable amount of space, just slow down by 2mph so they'll get bored and pass. If they insist on staying locked on your wheel, clear your nostrils. You don't have to aim because of the way a draft works.