They played a role, but Western institutions are biased to exaggerate it and gloss over the fact that the sanctions regimes were only adopted after the reinvigoration of the armed struggle fueled by stepped up material and training support from the USSR brought into serious doubt the survival of the South African regime even with continued active support.
The West didn't want South Africa to lose, and wouldn't concede to it losing until the alternative was it losing anyway and armed struggle and support of the Communist bloc being the entire narrative for why, which they didn't like for either international or domestic political optics.
It would have to work at coercing a final end to the 1950- Korean War before it could have a chance to succeed or fail at preventing another one.