LR doesn't have any tools that let you completely blur faces. The most you can do is use a local adjustment (e.g. the local-adjustment brush or the radial filter) and set local Sharpness, Texture, and Clarity to -100. But the effect is relatively small and you can't increase the effect by using multiple adjustments. You won't get the complete blurring I think you're going after.
You'll have to use Photoshop or a similar program.
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