Ask Mr. Smarty Plants

Pennisetum frutescens (Naked fountain grass) is a native of southern South America (Argentina and Paraguay) and it is listed of "Least Concern"  as an endangered or threatened species on the International Union of Conservation of Nature and Natural Resources (IUCN) Red List of Threatened Species.   There are other species of Pennisetum that are listed as invasive by the Global Species webpage, but P. frutescens is not so listed.  Notably, P. ciliare [synonym=Cenchrus ciliaris](Buffelgrass) is listed in the Texas Invasives list.  The suggestion given on their webpage is hand pulling and herbicides to control it.  P. alopecuroides (Fountain grass), another grass in the Genus Pennisetum, is listed as invasive by the National Park Service. Unfortunately, they do not give any control methods.  California Invasive Plant Council (Cal-IPC) lists P. setaceum (Crimson fountain grass) as invasive and has a thorough discussion of various control measures including mechanical, chemical and biological controls.  They do indicate that it is difficult to get rid of since its seeds are long-lived but they say that a combination of chemical and mechanical methods are the most effective.  Since I could find no specific recommendations for eliminating or controlling P. frutescens, I think your best bet is to follow the recommendations on the Cal-IPC webpage for P. setaceum.  They give detailed analyses of chemical methods and offer suggestions for mechanical removal.  You will need to be persistent in removing new growth by whatever means you choose.  When using chemical control, be cautious follow the safety instructions given with the chemicals to protect yourself, other plants you want to keep, and the environment.