I know morality does not block off gameplay but I was wondering if doing certain missions automatically earns me either paragon or renegade points. Are there any missions that are impossible to complete nicely and will mean I always earn renegade points for finishing (and vice versa) or will there always be a choice on how to complete it?
There will never be a point where you are forced to get renegade points (see the comments for a possible exception), or forced to get paragon points.
There are points where you have to pick one or the other, but never a point where you're offered only one option.
They changed this from Mass Effect 2 where at the end of a mission you'd get 5 points each. People complained that they wanted to play an entirely renegade or entirely paragon playthrough, but were forced to get some points in the other alignment.
They changed this system to getting neutral reputation now. The only time you get paragon or renegade reputation, is when you make a specific choice.
That will never happened with the new neutral reputations points.
However, you can considered that in the final mission if you are not enough Paragon,