Can you colonize a planet that has 100% blocked tiles

Stone Age primitives are actually a third state in-between the pre-sentient races that you can uplift and the fully sentient primitive civilizations that you can observe.

This is explained on the Stellaris Wiki's Pre-FTL Species Page:

There is a third possibility of non-FTL races, which are the stone age primitives. They appear as modifier on planets. If the policy for native interference is not Unrestricted, then the empire will be unable to colonise the planets. Therefore a pacifist or xenophile empire (or the fanatic versions) can't chose "unrestricted", and will be unable to colonise these planets.

So you might not be able to interact at all. Perhaps they will advance to the Bronze Age at some point and you can observe them. It's not clear if that's a possibility.

If you can colonize, though, here's what you can do once you get there:

Known Variations:

• Voluntarily retreat to Enclaves rather than contest the primitives for the land: Places a number of primitives on the surface map as tile blockers. Apparently these blockers cannot be removed, though they provide Social Science adjacency bonuses.
• Antagonize the aliens: fewer tile blockers
• Enslavement: No tile blockers. The new species appears as enslaved pops on the planet. The slaves can be emancipated, and integrated into the empire. Useful if a pop is neeeded to colonize worlds of the same type as their homeworld. Option is only available if slavery is enabled as a policy.

I think I actually had this happen on a planet in my current game, but I was confused because there was also a Pre-Sentient race on the planet. I uplifted that race, and was surprised to find that the Stone Age primitives continued to live in the reservations.

Apparently this bug is still present as of 1.1. If you're not concerned about achievements, you can use console to kill it:

• open console with ~

• type debugtooltip to enable additional info on tooltips

• find ID of the pop by hovering mouse over it, should be at the bottom of the popup

• type kill_pop <ID> into console