A. Mites love citrus plants but hate high humidity. Mist your grapefruit often during the winter, when the indoor air is dry, and if you can move it, take it outdoors for a quick simulated thunderstorm from your garden hose whenever the weather is above freezing. Mites also love heat and reproduce much more slowly in cool temperatures. If you can, keep your grapefruit on the cool side in winter, especially at night. If none of these measures keeps the mites under control, you'll have to spray them.
Nothing is more effective in killing mites than horticultural oil.
Oil kills all stages of mites from egg to adult, so one very thorough spraying might be all that is needed. Use it at half the rate specified on the label.