Just log into your account and go to my account and click on the "Edit" tab. On the Edit tab, Look down the page to the "Warm Showers Host Profile". There you'll find a checkbox for "Not Currently Available" (image). Click that box and nobody will find you via the map searches, and your profile page will say clearly that you're not available.
Please remember that this list is reciprocal hospitality, so you're expected to offer hospitality in the future (perhaps the distant future) even if you can't right now. (It's OK to go years, maybe many years, set to "not currently available". But someday...)