Oh, apologies to those who thought this would be about some lavatorial explosive prank, it's actually about that brownish-yellow limescale in the loo. Eeeurgh.
Best way to get rid of it? Citric acid: it's organic (rather than Organic), non toxic, bio-degradeable, and cheaper than "limescale removers".
And it crops up in bath-bombs, combined with sodium bicarbonate, to produce that appealing fizz.
For a 'loo bomb', rather more citric acid and less sodium bicarbonate than the bath version, plus a pleasant, bio-degradeable scent of your choice. So, when it's time to de-scale, plop it in, enjoy the fizz, and a sparkling, freshly scented loo awaits you next time.