We present an approach to completely stop terahertz radiation in an optical system with a gyroelectric semiconductor. This system is composed of guiding and stopping parts formed by the semiconductor with different cladding structures. Because the dispersion properties of surface magnetoplasmons (SMPs) in the semiconductor closely depend on its cladding structure, robust one-way SMPs sustained by the guiding part are prohibited in the stopping part, thereby stopping terahertz radiation without any backscattering. For incident continuous waves, trapped spots with strongly enhanced fields occur on a subwavelength scale. For incident pulses, the wave packets can be completely trapped and simultaneously compressed to subwavelength sizes.