How far from your primary address are you willing to invest in.

I called the tax assessors office - hoping to find the address of the absentee owner.

You must be firm, fair and swift in addressing this issue.

There are ways to prevent this by account and password, MAC address tracking, or limiting bandwidth which most of the large ISP already utilizing.  

My email address is [email protected] and my phone 347-210-5974. 

Did you give a change of address, or was the unit in question the address you had with the association.  

I even get them to sign a "Return of Possession" form after they have vacated and returned the keys; on this form is a spot for their forwarding address. 

When purchasing a property with tenants in place, there are some common procedures to take that are discussed at length in other posts; use "search the site" in the right hand side of the blue bar at the top of this page to find more on this topic.As @Joel Owens says, the best time to address some of these problems is before the closing.

I would recommend you work on getting to know realtors so you can either ask them for comps or at least an analysis of the area.To address your immediate need, here is what I used to do: Visit Zillow, Trulia, Eppraisal, City/Town GIS (to get appraised value) and RealQuest Express.  

Get a formal education first, buy a text book for RE Agents at an RE school and study that, if you have a question from the text book mention me and I'll be happy to address it.