We propose a novel method to actively control the propagation of THz surface plasmon polaritons in plasmonic waveguides. This method relies on the excitation of long range surface plasmon polaritons (LRSPPs) in thin slabs of semiconductors. The characteristics of LRSPPs (electromagnetic field confinement and mode propagation length) can be precisely tuned by optically pumping charge carriers to the conduction band of the semiconductor. This excitation can be local by illuminating a flat semiconductor slab with an expanded optical beam that is structured with the shape of the THz waveguide. In this way, it may be possible to design active and integrated THz circuits by simply changing the illumination pattern on the surface.