First, a quibble: in completely empty space, the speed of a spacecraft is ill defined. What would it be moving relative to? Empty space?
Now, let's say that the spacecraft is moving relative to the CMB at 0.5 c in otherwise empty space. I'd like to say that no thrust is required because there is no drag on the spacecraft to decelerate it. However, due to the blue shift of the CMB in the direction of travel, I suspect that there may actually be some radiation pressure decelerating the spacecraft.
Another minor quibble: Mass acting on other masses is a Newtonian conception of gravity. In the context of GTR, mass does not "act upon objects"; mass (energy) "acts" on (curves) spacetime. This curvature is related to the density and flux of energy and momentum which are frame dependent quantities (though they are expressed in a frame independent way via the stress-energy tensor).
The bottom line, as Ron points out, is that, in the coordinate system in which the rocket moves at 0.5 c, it is the relativistic energy and momentum of the spacecraft that relate to the spacetime curvature as "seen" in that coordinate system.