Flax bots are macros that run back in forth between Seers Village bank with flax. They're rather easy to spot because they usually have the default look. If you feel that you are being cheated out of your resources and have enough evidence to prove it, report it via the 'Macroing' option under Honour.
Flax bots can also be found in Lumbridge Castle spinning the collected flax into bow strings, which they then sell for runescape gold and sell the collected gold for real world cash (though they may also just sell the bow strings for real life money).
Flax bots could also be killed by luring them to the giant bats next to the Keep Le Faye, south east of the flax field. It involved you dressing up in dark green clothes (Fremennik robes work best, which are bought from Yrsa's Accoutrements in Relleka), and standing outside the entrance to the field. After a while, a bot would walk up to the luring player, thinking they are flax (since flax and the robes share the same color pixels, which bots use to detect and pick flax). The player would then take a few steps towards the keep, and the bot would follow them, trying to "pick" them. The player would then continue luring the bot to the keep, where a giant bat would attack the bot and kill it. This is no longer possible due to an update which abolished real world trading from the game, making most flax bots disappear.