You need to make sure they do not contain any tables

Please run this query

select 
    table_schema AS BlankDatabases 
from
    (select 
        A.table_schema, COUNT(B.table_schema) table_count 
     from
        (select 
            schema_name AS table_schema
        from 
            information_schema.schemata
        where 
            schema_name not in ('information_schema','mysql','performance_schema')
        ) A
        left join
        (select 
            table_schema
        from 
            information_schema.tables
        where 
            table_schema not in ('information_schema','mysql','performance_schema')
        ) B
        using (table_schema)
    group by 
        A.table_schema
    ) AA 
where 
    table_count = 0;

This will tell you which databases do not have any tables

For example, let's say you run that query and get the following output

mysql> select table_schema BlankDatabases from
    -> (select A.table_schema,COUNT(B.table_schema) table_count from
    -> (select schema_name table_schema
    -> from information_schema.schemata
    -> where schema_name not in
    -> ('information_schema','mysql','performance_schema')) A
    -> left join
    -> (select table_schema
    -> from information_schema.tables
    -> where table_schema not in
    -> ('information_schema','mysql','performance_schema')) B
    -> using (table_schema)
    -> group by A.table_schema
    -> ) AA where table_count = 0;
+----------------+
| BlankDatabases |
+----------------+
| junk           |
| test           |
+----------------+
2 rows in set (0.00 sec)

mysql>

You can just run

DROP DATABASE junk;
DROP DATABASE test;

In your case, you should surround the database names with back quotes (`)

DROP DATABASE `@feff...@feff`;

If you are having a hard time, go to the OS and remove the folder.

For example, if your datadir is /var/lib/mysql and you are removing @feff...@feff, do this

cd /var/lib/mysql
rm -rf "@feff...@feff"

GIVE IT A TRY !!!