If you are having issues dropping a postgres database the issue is more than likely that a user is accessing the database you are attempting to drop. There isn’t technically a way to force drop the postgres database however there are steps you can take to prevent others from accessing the database and if needed kick the users whom are accessing the database.
If you receive an error similar to the below the issue with your attempt to drop the database is the fact that either a user is accessing the database or an application of some sort has an open connection to the database. In the second scenario PostgreSQL considers the application the same as a user. In the code below the name of the example database is dev.
Please note that dropping the database will destroy all data located within the database. I strongly suggest always backing up your data before dropping the database. You can backup the data with pg_dump and regardless of if you think you will need the data or not it is best to keep a copy just in case.