The rate-limiting commands mentioned above should help; if your firewall is using Gaia though make sure the connections table is set to Automatically as shown, you should not run out of connection table slots unless Gaia itself runs out of physical memory. If you upgraded from an IPSO or SecurePlatform-based firewall this may still be set to the manual limit of 25000.
In my book I cover this exact scenario in the context of a nemesis-worthy internal auditor named Jim Profit doing port scans through the firewall.
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