Never heard of red sorrel. There are common sorrel, sheep's sorrel,mountain sorrel and French sorrel - perhaps you mean one of these? French sorrel is a garden herb and all green, so probably not that. Common sorrel has a red flower spike so perhaps that is what you mean. Just dig it out. You can use the leaves in spring to flavour soups and stews, particularly good with lentil and tomato. (A strong lemony flavour)

Red sorrel (sheep's sorrel) is a shallow yellow taproot, combined with multiple rhizomes, so you need to dig it up and get the roots out. Or apply an effective weed killer. Spreads by seeds also so dont put any in the compost.