can u abandon a colony planet, if u can, how do u do it....

Yes you can.

Click on the planet so that you are working on that planet. Above the picture of the planet you'll see Planet "what ever you named this planet"

Click on that and you can change the planet's name or abandon the colony with your password.

Once you abandon your colony, what happens to the buildings you had on it?

They are destroyed with the planet. Same for any resources on the planet. That's why you ship the resources to another planet.