There is no best way to kill bentgrass. The method you choose will largely depend on the type of grass growing in your lawn and personal preference. However, whichever method you choose, you must follow it with effective cultural practices. Only lawns that are well-watered, fed and frequently mowed can stand up to subsequent bentgrass invasions. Thin, poorly kept lawns have to be weeded season after season.
Use a trowel to dig up individual patches of bentgrass and their roots. This may sound like backbreaking work, but it is the option with the least threat to the surrounding lawn. The herbicide that kills bentgrass also kills the grass surrounding it.
Spray each patch of bentgrass with a non-selective herbicide such as glyphosate in the spring or summer when it is growing. Non-selective herbicides will kill any plants it comes into contact with, so they must be applied carefully. Use a hand sprayer to spray each patch of bentgrass until all of its foliage is coated but not dripping.
Paint each patch of bentgrass with a non-selective herbicide in spring or summer when it is growing. There is always a risk of damaging the surrounding grass when you spray herbicide. Another option is to dip a medium-sized paintbrush into the herbicide and paint the patches with the herbicide. Be sure to coat all the blades but not to the point of runoff. Once the bentgrass is dead, dig it up.
Replant bare patches left behind by digging or herbicidal spray (first wait the amount of time specified by the herbicide's manufacturer). Use a trowel to loosen the top 2 inches of soil. Then scatter grass seed over the bare patch (this is not an exact science; one centimeter or so between grass seeds is sufficient). Walk over the seeded patch to ensure the grass seed makes firm contact with the soil. Water so the top 2 inches of the soil is moist. Keep reseeded patches constantly moist for two weeks.