The hotter the climate you live in, the more trouble yiu’lol have with this. Warm soil causes the plant to flower. Mulch it well to keep the soil cool. Once any flowers appear, allow the plant to go to seed. The seeds produced are coriander, which can also be used. This site has some interesting ideas about how to deal with bolting, such as getting slow-bolt seeds and planting the seeds several weeks apart so you always have a fresh crop.Bolting Cilantro – Why Does Cilantro Bolt And How To Stop It

• Plant cilantro outdoors immediately when the outdoor temperature is still below 23 degrees C. Cilantro requires at least eight to 10 weeks of temperatures below 23 degrees to produce the most foliage before it begins to bolt, and any warmer temperatures will induce flowering.
• Cover the soil around the plants with a 2-inch layer of mulch to help keep the soil temperatures cool and moist, which prevents early bolting.
• Water the cilantro about once weekly when the top 1/2-inch of soil feels dry, providing 1 inch of water or enough to moisten the top 6 inches of soil.
• Pinch back the tips of each upright stem when the plant grows to a 4- to 6-inch height. Cut off the outer leaves when the leafy stems are 4 inches long. Frequent harvesting and pinching keeps cilantro compact and slows bolting if the temperatures remain cool.