What's the proper way to deal with someone who Team Kills and harasses you constantly in Starcraft 2?

Sadly there is no method by which you can prevent being matched with another player without breaking ToU, however Team Killing is a form of harassment and can get you banned if reported. I would suggest you contact blizzard support as soon as possible.

Webform: Email Form

For detailed information on how to contact Account Administration visit the Account Administration page.

For assistance with StarCraft II in other regions such as Europe, Korea, or China, please refer to our International Support Contact page.

About contacting Blizzard (from Blizzard Support page):

Webform: Email Form

For detailed information on how to contact Account Administration visit the Account Administration page.

For assistance with StarCraft II in other regions such as Europe, Korea, or China, please refer to our International Support Contact page.

So it looks like you can send a complain about harassment by filling out their Email Form.