How do I actually get rid of spider mites rather than just knocking them back?

Spider mites fail to clear for one reason: the treatment interval does not match the life cycle. Eggs are unaffected by most sprays, and at typical indoor temperatures they hatch in three to five days. A single thorough spray kills the adults and nymphs, then a fresh generation emerges and you conclude the treatment did not work.

Treat on a schedule instead. Spray thoroughly, repeat at day four, again at day eight and once more at day twelve. Four applications spanning at least two full hatch cycles is what actually breaks the population.

Coverage matters as much as the product. Mites live on the underside of leaves, in leaf axils and along stems. Anything you spray must reach those surfaces, which usually means tipping the plant, spraying upward, and doing it somewhere you do not mind getting wet. Insecticidal soap and horticultural oil both work by contact and smothering — they have no residual action at all, so a surface you miss is a surface that survives.

Neem oil is slower but adds an antifeedant and growth-disrupting effect. Emulsify it properly with a drop of mild soap in warm water or it will separate and scorch leaves, and never apply it to a plant standing in direct sun.

Isolate the affected plant immediately, and check every plant within a metre of it — mites disperse on air currents and on your sleeves.

Prevention is mostly about conditions. Mites thrive in warm, dry, still air, which describes most heated flats in winter. Raising humidity and rinsing foliage under the shower once a month makes an infestation far less likely to take hold in the first place.