Ask Mr. Smarty Plants

Since your question was about getting rid of them, the nativity really isn't the big issue. A weed is a plant where you don't want it. If the plant is native to the state where it is, that isn't necessarily a free ticket to stay there. However, whoever planted those agaves was justified in that they are native to California, and native close to the area where they were planted. They are desert plants and if they have found a very comfortable place to grow, they could be characterized as invasive. Bottom line: they are there, they are native there and getting them out is going to be very difficult. Short of a time machine whereby the planters could go back and not plant them, your choices are: get permission to have them removed (if that is necessary), pay a contractor to move them (and you will still have "pups" no matter what you do), or do it yourself. They are tough survivor plants, evolved to survive in a difficult environment and there is no spray you can buy at the home improvement store that will solve the problem.