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You don't mention whether you overseeded an existing lawn or seeded a new one. It would also help to know if you are growing it in sun or shade.

If you seeded a new lawn and the grass is just at the young seedling stage, starving it of water will kill most of it. You could also smother it under black plastic or spray it with a grass killer or a non-selective weed killer such as RoundUp or AllDown (which is organic).

If you overseeded a lawn, you have few options. You cannot use RoundUp or a grass killer without harming the rest of the lawn (although a low dose of grass kiler might kill more of the fescue, which is young, than the established lawn).

If the overseeded lawn is more aggressive than tall fescue, it might choke out the fescue. Or if the overseeded lawn has a recommended mowing height lower than tall fescues, mowing often to the height of the main lawn might also weaken the fescue.