You might find these useful:
www.harcourt-brown.co.uk
Downloadable recommendations for feeding rabbits with urinary tract disease
www.harcourt-brown.co.uk
There are other pages that may also be of interest, but these are the main ones relevant to reducing calcium.
The usual advice is to stop feeding anything that is obviously high in calcium (FHB has a list amongst those articles), offer water in bowls to encourage greater uptake, and try to encourage them to move about more so that they empty their bladder more fully. A lot of pet rabbits are quite sedentary and the sludge just sits there and tends to build up, causing problems. Wild rabbits running through fields keep the bladder contents well mixed up and emptied more thoroughly (as well as their natural diet generating less excess calcium).
Fresh greens can be fed wet in order to increase water consumption. Fresh grass contains a lot of water compared to hay, if you have access to any. More water intake from any source (ie wet food or drinking water) means more urine output, therefore better flushing out.
I've always found that certain rabbits are a lot more prone to bladder sludge than others, and that cutting back on anything that makes it worse is the only long term option to control it. There isn't an instant fix here - it's a lifetime change of diet. Alfalfa hay should be avoided - standard grass hay is fine and should be encouraged for general gut and dental health. Cabbages and other brassicas are often an issue with my rabbits that have had sludge, so swap for something else (eg lettuce). There are lots of suitable greens that you can feed, and it's worth looking at forage.