I usually save seed from mine to establish in areas where I need it. If you don't want it to spread, I'd suggest not letting it flower or lay along the ground where it can set
roots. I have the stuff everywhere but I like it because it is a good insect repellent as well as being a good
polyculture buddy for fruit
trees. When I mow it around the rock walls of my herb garden, the smell is pretty awesome.
Other herbs that have done the same thing for me are oregano, mountain mint, sage, thyme and camomile.