Drop '\user' - slash user from database
Try putting the name in brackets.

DROP USER [\username];


Given the problems you are having try doing this through dynamic SQL just to confirm that you are catching every possible character.  You could also try this through Object Explorer.

DECLARE @sql nvarchar(1000)
SELECT @sql = N'DROP USER [' + name + N']' FROM sys.database_principals WHERE name like '\user%'
PRINT @sql
EXEC sp_executesql @sql


@solomonrutsky brought up an interesting question.  Try running the following:

SELECT s.name AS Sysusers_Name, dp.name AS DP_Name
FROM sys.sysusers s
JOIN sys.database_principals dp
    ON s.sid = dp.sid
WHERE s.name = '\user'


See if you get any results, and if so if the Sysusers_name doesn't equal the DP_Name.  If you get a DP_Name try dropping that one instead.