Surface plasmon polaritons (SPPs) propagating along a metal surface corrugated with an array of shallow grooves filled with dielectric are studied analytically. It is shown that the asymptotic frequency of SPPs can be greatly decreased by stuffing the grooves with dielectric, and thus the fields exhibit much better confinement. In the THz frequency range, the influence of the absorption in real metal on the performance of spoof SPPs is also examined. The numerical calculations show that long propagation length and subwavelength field confinement are simultaneously available for spoof SPPs on the real surface corrugated with grooves stuffed by the dielectrics. The good guiding ability of the structured surface at THz frequencies is further verified by numerical simulations of wave transmission. The proposed structure may lead to a new design of devices for routing terahertz radiation in highly integrated circuits.