If you need to do it in psql or using a postgresql client you could issue this query to stop those sessions:

SELECT pg_terminate_backend(pid) FROM pg_stat_activity WHERE datname = 'db_name' AND pid != pg_backend_pid();

Change db_name for your target database to drop

But Odoo itself do this if you drop the database through Odoo database manager ui accesible at:

http://localhost:8069/web/database/manager#action=database_manager

In the menu drop, select the target database to drop, type the instance admin password and clic drop button or press enter.

Change localhost:8069 in the previous url to match your Odoo instance url