This mod i linked has option's like "time set" where you set period of time for example 60minut and if same user posts 2 msg in row in this time they got marged into one post. You can set this to 6h or 12h and problem is gone

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Same way about flood and spam bots, there are mods for this and they are easuly to install(acp in this smf version has in global configuration anti spam/flood controls just need to set them up, change 0 to 1(off to on) and set the time limit between new threads)