also you want to be using the sys.database_principals catalog view and not sysusers. This should be better

declare @sql nvarchar(max)

set @sql = ''

SELECT @sql = @sql+

'

print ''Dropping '+name+'''

execute master.dbo.sp_revokedbaccess '''+name+'''

'

FROM

sys.database_principals

WHERE

name NOT IN('dbo','guest','INFORMATION_SCHEMA','sys','public')

AND TYPE <> 'R'

order by

name

execute (@sql)