Actually, of interest Wikipedia seems to say that you wouldn't have to leave Earth at all (just bring the other half of the wormhole back), so I think you're right that it doesn't solve the grandfather paradox: https://en.wikipedia.org/wiki/Wormhole#Time_travel
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In 1978 the other you would be 1 year into their light-speed trip away from Earth. How can you kill them?
As mentioned in other comments, another wormhole or a more direct path to cut him off.
Actually, of interest Wikipedia seems to say that you wouldn't have to leave Earth at all (just bring the other half of the wormhole back), so I think you're right that it doesn't solve the grandfather paradox: https://en.wikipedia.org/wiki/Wormhole#Time_travel
Cool, seems we've reached the bottom of this one!