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You fire them as of a date two months from now. For those two months they are technically still employed, but do not have to come to work unless asked. Just like when you resign with two weeks notice, you tell them you're going to end your employment in two weeks and you're still employed until then.

If the employee finds another job, they should tell them when that job asks "when can you start?" a date after the two month severance period is over. It's not that complicated.