Pampas? These are an invasive in California, so there's a lot of institutional information on removal.

From UC Davis:

Mechanical control Pulling or hand-grubbing jubatagrass seedlings is highly effective. For larger plants, however, a Pulaski (axe on one end and hoe on the other), mattock (pick on one end and hoe on the other), or long-bladed shovel is the safest and most effective way of removing established clumps. To prevent resprouting, it is important to remove the entire crown and top section of the roots. Regrowth from the lower roots is unlikely. Detached plants left lying on the soil surface may subsequently take root and reestablish under moist soil conditions; therefore, some weed workers recommend turning the clump upside down so the roots are exposed to the air. A large chainsaw or weedeater can expose the base of the plant, allow better access for removal of the crown, and make disposal of the removed plant more manageable. Once flowering has occurred, cutting and removing or burning the inflorescence is important to prevent seed dispersal during the mechanical operation.

Chemical control Control of pampasgrass and jubatagrass can be achieved by spot treatment with a postemergence application of glyphosate (Roundup Pro) at about 2% solution or 8 qts. per 100 gal. Homeowner formulations of Roundup will require a 4% solution. The addition of a nonionic or silicon-based surfactant may be necessary to enhance foliar penetration of the herbicide. For most effective control at a 2% solution, plants should be sprayed to wet but not to the point of runoff. Summer and fall applications were both effective, as photosynthetic assimilates are translocating downward at a faster rate late during these seasons. In some cases it may be necessary to apply the herbicide prior to the maturation of viable seed in late summer. In addition to the standard 2% spray-to-wet application of glyphosate, a low volume glyphosate foliar treatment at 8% of product or a 33% solution of concentrated Roundup using a ropewick technique can also give excellent control. The low volume treatment has been shown to be the most cost effective. The graminicide fluazifop (Fusilade) applied as a low volume 4% (product) application in the fall also gave good control of jubatagrass.

In forestry operations, hexazinone (Velpar or Pronone) is a soil residual root absorbed compound also effective in the control of pampasgrass. Once plants have been killed, clumps can be removed mechanically and left to decompose naturally. In wildlands, leaving the dead clumps may prevent subsequent pampasgrass or jubatagrass seedling establishment.

Combinations

For very large clumps, cutting or burning can remove the top foliage and the regrowth can be treated with a systemic postemergence herbicide. This method reduces the amount of herbicide applied compared to treatment alone.