They most certainly can be guided. Ballistic means that the terminal phase of flight is not propelled, eg they are just falling and no longer accelerating.

Guidance can be inertial, magnetic, radar, GPS, or even star-map for ICBMs, or a combination of many of these. Minuteman missiles still rely on the position of some stars to orient themselves in exosphere coast phase. Part of their flight may be unguided, namely 3rd stage ascent where they are spin-stabilized, and during reentry phase where atmospheric plasma makes GPS guidance impossible and inertial might be inaccurate.