A solution for spring onions is to mow them and let them regrow clear. It may be worth trying with garlic too. An organic remedy ( found on the net so not tried personally) is

A heaped tablespoon of wood ash is stirred vigorously into a litre of water, left to stand overnight, strained and mixed with a cup of sour milk or buttermilk. Before spraying the mixture is diluted 3 times with water, but the strength of the solution should be checked by a trial on a small area of plants to ascertain the most effective dilution.

Rust on leeks is not related to rust on onions. Leek rust is often associated with high nitrogen and low potassium, so a potassium rich fertliser ( such as many tomato feeds ) can be useful.

A 1999 trial found that non US sourced garlic was less susceptible to rust.