Go to Settings → Cloud Data in the SiderMem extension, verify your identity via a one-time email code, and clear all synced memories. Local data is never touched by this — cloud data is permanently purged from SiderMem's servers within 7 days of the request.
Local memories in your browser's IndexedDB are completely untouched by a cloud data deletion — see where your data is stored for the full local vs. cloud breakdown.
You don't have to clear everything — deleting individual synced memories from the side panel removes just that one from the cloud and immediately frees up quota. The Cloud Data setting is for clearing all synced data at once.
Deleting cloud data is irreversible after the purge window, so it requires the same email one-time-code verification used for signing in, to make sure only you can trigger it.