You must drop the distribution against a published database

The regular script is the following:

Use [master]
EXEC sp_dropdistributor @no_checks = 1

If this message appears:
Cannot drop the distribution database ‘distribution’ because it is currently in use.

Then you can correct that by using the following script:

ALTER DATABASE Distribution SET SINGLE_USER WITH ROLLBACK IMMEDIATE
Use [master]
EXEC sp_dropdistributor @no_checks = 1, @ignore_distributor = 1
GO