Relativistic Electrons (REs) pose a formidable risk to future high-current tokamaks such as ITER and SPARC. Any effective mitigation strategy must seek to reduce the maximum RE energy since that determines the potential for damage deep inside plasma-facing components. Resonant wave-particle interactions provide such a means of phase-space control, since pitch-angle scattering of REs increases the synchrotron damping they experience and hence limits their maximum energies. Externally-launched helicon waves (i.e., whistler waves, fast waves in the lower-hybrid range) are observed to limit the growth and maximum energy of REs in low-density Ohmic DIII-D tokamak plasmas. Owing to the low density of these plasmas, an appreciable RE population forms after 1-2 seconds. The waves are then launched anti-parallel to the plasma current, so that the normal Doppler-shifted wave-particle resonance condition with REs of ~8 MeV is satisfied. Following the application of helicon waves, the synchrotron and non-thermal electron-cyclotron emissions, both strong functions of the perpendicular energy of the REs, increase. The total hard x-ray emission, a proxy for the RE population, ceases to grow. Energy-resolved hard x-ray measurements show a striking decrease in the number of high-energy REs (> 7 MeV) to below the noise floor and a significant increase in low-energy REs (< 4 MeV). These observations strongly support the hypothesis that resonant REs are pitch-angle scattered by the waves and then, because of an increase in synchrotron emission, are damped down to lower energies. This occurs despite the toroidal electric field remaining high enough to drive exponential RE growth in the absence of helicon waves. These results open new directions for limiting the maximum energy of RE populations in laboratory and fusion plasmas.