Is there a way of preventing a brain tumor after years of heavy mobile phone use or will the damage already be done?

Unclear: Most of the studies linking cell phone use with brain tumors are poor studies and the current stand of both neurosurgery associations is that there is not enough data to suggest there is a link between cell phone use and brain tumor. If there is such a link, then decreasing your use may improve your chances but again, i suspect there is not direct causative link between cell phone use and tumors.

Unlikely: All evidence we have so far suggests that cell phones do not cause brain tumors.

No conclusive : Evidence. There is no conclusive evidence that mobile phone use causes brain cancer. So do not worry. In any case there is no known preventive action anyway.

No proven danger: There is no convincing scientific evidence to suggest that cell phone use, heavy or otherwise, causes brain tumors. This has been looked at in several major studies, and there is neither a plausible mechanism nor clinical evidence of a connection. Don't worry about this.