I know from from personal experience that a restrictive covenant can certainly exist after the person leaves. They are routinely used for that purpose; for example where a business owner sells a business (and the new owner doesn't want him to set up in competition) or where a key employee leaves and then the RC is used as a negotiating point (the more he agrees to not do, the more you give him as a golden handshake
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The RC has to meet various requirements if it is to stand up though and expensive specialist advice is needed. Employment law is an absolute nightmare and only those bang up to date are any good. I can't recall the details but e.g. there has to be a reasonable time limit, and you can't prevent somebody using their manual or intellectual skills to earn a living.
The best way to trap former employees is to prove that their new product was designed using tools developed in, or owned by, their former company - software tools are an obvious one (a bootleg copy of a C compiler, or even a windoze installation CD). That's why when you start your own business, having worked somewhere previously, you must purchase all new software tools, PC, printer, etc and keep the receipts. Even a text editor has to be purchased afresh (I remember purchasing a copy of Brief for £300
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You can prevent the ex employee canvassing your former customers but this is much more difficult. It has to be done by proving he walked out with company confidential data. All he has to do is purchase a mailing list from some market research agency, which just happens to have all the right names on it, and he can mailshot the lot, and he's covered
In reality nearly all programmers walk out with the sources of everything they have written while working for you - this is to be expected.