In that case here is a very easy to use Transact-SQL script that you can use from any backend you want:
1. First; Changed the owned Schema (just the same as SSMS suggested by @isoftech above) using the ALTER AUTHORIZATION instruction something like this:
The instruction above changes the ownership of the Schema named
"TheSchemaToAlter" to the owner named
2. Afterwards you can safely drop the user using DROP USER instruction:
DROP USER TheUserYouWantToDelete
This solution should work perfectly for you, Remember that an User might have ownership on multiple schemas so make sure to Alter Authorization for all those schemas.