you'll have to do both the remove and ignore operation
- first make a backup of your local file (like @ibz said)
- then remove the web.config from the repository.
- then copy back the web.config to the same folder
- finally use svn:ignore so that subversion does not try to add it again to the repository
since i use tortoisesvn i can't really tell you what svn commands you have to use, but using tortoisesvn it would be:
- make backup
- right click on web.config on the folder under source control, select TortoiseSVN | Delete
- right click on web.config on the folder under source control, select SVN Commit => after this you will notice that the file is actually deleted from the file system
- move up and right click on the folder under source control, select TortoiseSVN | Properties
- on the properties window click new + property name "svn:ignore"; property value "web.config". accept changes
- commit changes
on my .net projects i include the following exclusion with svn:ignore: bin, obj, *.suo, *.user