How can you prevent lymphoma?

You can't. There are no known preventative measures other than staying away from high doses of radiation and known carcinogens.
Not much you can do. Most of the times we don't know what exactly causes lymphoma so there isn't much you can do to prevent it. Most cases probably have a genetic component but the process is probably multifactorial and many other things need to happen in order to produce lymphoma. In a small subset of cases, a distinct cause appears to exist: exposure to toxins, radiation, certain viruses (ex: ebv).