The problem with spider mites is their eggs: they are extremely resistant and may remain "dormant" for months, waiting for the right conditions to hatch.
The best cure against spider mites and their eggs is mineral oil: spray the plants throughy with them and do not wipe them. Mineral oil form a thin layer over insects and their eggs that will smother them. They cannot build resistance to lack of oxygen. Eggs will fail to hatch and eventually die and rot.
Mineral oil has no expiration date, is non-toxic (unless you drink it straight from the source), works against a large range of pests and if applied correctly harmless to bees so it's something always worth having around.
