Welcome to our Forum, Janna!
I'm not aware of a way to delete acquaintances from the relationship section. However, I think they might fall off on their own eventually if your Sim doesn't talk to them again.
To specifically find out if a Sim is married, click on the Romantic interactions tab and choose "Ask if Single". If the Sim is married, it will tell you the name of the spouse. Other than that, you generally won't know the relationships among the townies until you just get to know them. If anyone marries into your household, you'll be able to view the family tree, which will give you more information about that particular Sim. The
Sims Wiki also has a list of all of Sunset Valley's townies, which might help you, as well. Scroll down until you find Sims 3.