Strong smelling plants taking over garden.
This is called Houttuynia (Houttuynia cordata). We get frequent desperate requests for help with this plant because of its invasive nature. Once you have it, it spreads readily by underground roots (rhizomes) and it is very difficult to remove if it gets into areas where you don't want it.
Usually, it is sold in a variegated version known as 'Chameleon Plant.' But, after a few years, it reverts to its original color. Either way, it rapidly becomes obnoxious and nearly impossible to get rid of.
If you have no other plants in the garden bed and you want to remove this plant, you can try hitting it with glyphosate, a nonselective systemic herbicide (that kills the roots) and then hand dig the plants that the chemical does not kill. Glyphosate is found in products such as Roundup and will kill all types of plant material, so you must be very careful to not apply it to the lawn or other desirable plants. It will probably take several applications to have any noticeable effect on the Houttuynia.
If the Houttuynia is mixed in a bed with other plants, it is riskier to use chemicals because you may kill the desirable plants. You have a few options. You can hand dig the Houttuynia. Again it will take some time to get all of the plants and their rhizomes. You could try painting the glyphosate on the Houttuynia with a small foam paint brush. Or spray, but use a shield of cardboard or plastic to keep spray off desirable plants. By doing this you would be less likely to apply the chemical on the desired plants. Then hand dig any Houttuynia that is not killed by the chemical.
A last option is to remove the desirable plants to another bed and use the chemical and mechanical method described in the above paragraph. If you decide to do the last option, be very careful not to move any of the Houttuynia rhizomes to the new bed. You may want to wash or shake almost all the soil off the plant roots to be sure no houttuynia is entangled. These rhizomes could very easily be mixed with the desirable plant roots.
Here are links to a couple of sites that discuss Houttuynia removal.