There is only one easy way and that is an air rifle or .22. If you can't use that then use one of the many excellent kill traps on the market. Sentinel or trapinator traps are MUCH better kill traps than a timms. While there are some interesting solutions to killing a possum mentioned above, most of them will be hard to justify in a public situation.
As an aside, I can concur with Micky Duck on the how hard a possum can bite!
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