Logging out of the app (and then logging back in) should do the trick. Your observations will still be stored on the iNat servers, but they won’t also be taking up memory on your phone.
One caveat: If you have any observations pending upload, make sure to upload them before logging out. Otherwise you’ll lose them.
You may also be interested in a feature request that allows clearing the cache: Limit Cache Size or Allow Cache Clearing for Apps - Feature Requests - iNaturalist Community Forum