You could give the planets to a friendly AI or, on the planet screen, click 'details' then click 'disband colony'.

Disbanding colonies in 1.3+ sets the class to 0 so the world cannot be reused by anyone.

This also works as an extremly effective "scorched earth" policy if a war is not going well.

Man, that's harsh!

I usually just take tha crappy planets and give them a stock exchange, however many fit. No factory , maybe a farm or morale Improvement if I have room. Once built, set them to research focus for the few extra points and let them make money. Don't usually even protect them in anyway(not worth the effort).

I used to give them to minors, whether I was allied to them or not. I did the same thing with my obsolete ships. Pretty soon you've got a major minor race except they don't do much, which is a pity!

Put stock markets on the lowgrade planets. Rush by them if/when you have the BC or they will take forever to build.

There is no such thing as a worthless planet. At least not to me. In long range wars where the border between you and your enemy doesn't abut, those captured "worthless" worlds are your beachhead. I don't like just capturing a planet in someone else's influence zone. If I invade a sector, I don't stop until my influence dominance kicks in. In close in worlds, they represent additional influence that keeps enemies from bogging your border worlds in their territory of influence.

Usually when I go to war, the first thing I do is drop my Social Production down to 10-15%. So even if they do have social projects going on, there's likely not enough resources in the pool to assign to their world anyway, so they're not a drain until you make them so when the war is over.

If the PQ is low, I invade wih Mass Drivers then abandon the colony.

You could give the planets to a friendly AI or, on the planet screen, click 'details' then click 'disband colony'.

Disbanding colonies in 1.3+ sets the class to 0 so the world cannot be reused by anyone. That sounds like just what you need

... Um... right. This sounds like Mass Drivers would be a waste of money if abandoning/disbanding reduces a planet to PQ0.

Disbanding colonies in 1.3+ sets the class to 0 so the world cannot be reused by anyone.

Cute - I never do this, but it's nice to know if the need ever arises.

I thought I must have missed it in the change logs, but I just checked and this is an undocumented feature.   

I've disbanded colonies before. I don't do it often, but mainly when I don't want a planet for some reason and don't want another race to use it. Most of the time, I just throw a few stock markets on those low PQ planets I commandeer. They do make some money so it's worthwhile. It's more productive than destroying them. In any case, it's good to have the option. It can be useful sometimes.