If you're putting route language (o:DL+) in for every leg and getting non-DL flights back, can you provide us with an example? I'd like to look into this. Easiest is to press the "comments/questions" button at the top of the results page where you saw that, and fill in the form.

Anyway, if you want to eliminate codeshares completely, you can do this:

From: BOS::DL+ / -codeshare
To: MSP::DL+ / -codeshare

That will give you DL-marketed flights, and only the ones actually operated by DL.