Fill Count For 14/3 G NM With 'Unused' Red
1 unit per black wire. 
1 unit per red wire.
1 unit per white wire.
1 unit for grounds.
1 unit for stud or hickey (if that's what you're going to use to attach the fixture).


That's 5 total units, times 2 cu. in. per unit (since you're using 14 AWG conductors).  That's a total of 10 cu. in., which is well over the 6 cu. in. the box is rated for. Even if you attach the fixture directly to the box, the fill calculation would be 8 cu. in.

NOTE: Above count is based on a single conductor of each color. If there are more than one, you have to count one unit per wire.

It makes no difference if the wire is live or not, all conductors that enter the box are counted.

And yes, according to 314.17(A) the unused knockouts should be closed.



On a side note. Grounding conductors don't count for 1 cu. in., they count as 1 unit. That unit is then multiplied by the value from Table 314.16(B), based on the largest conductor in the box. So in your example, all the grounds require 2 cu. in., because you're using 14 AWG conductors.