it is true for any material thing that one measures to be moving at less than c relative to one's self. For such a thing, like a spaceship, there is no physical process that can accelerate it to greater than c. The flip side to this is the weird science of tachyons, which are hypothesized particles moving at greater than c relative to your frame but which have never been observed.
Nonetheless, the same physics that prevents moving at greater than c relative to you prevents anything observed moving greater than c from decelerating to c or below. In the weird, topsy turvy tachyon world, tachyons lose energy by increasing their speed, so they tend to accelerate off to infinite speed, which maybe why none have stuck around for us to see. That whole business is truly weird, and Selena Ballerina is a bit weirded out today, so i'll deal with in another article.