There is no way a source control system can stop you from editing a file that is in your computer. Only thing the source control can do is, make the file readonly to remind you to checked out the file.
You can always go to the windows explorer and change the ‘Readonly’ attribute of the file. This will allow you to edit the file!
But not that by doing this, you are cheating the source control. You cannot checkin the file you changed (since you have not checked out the file). If you somehow manage to checkin the file to source control, you may end up messing up the file. Some other members may have changed the same file and you may overwrite his changes by checking in your version of the file. (If you had checked out the file before you changed, then nobody else can checkout that file and change anything which will ensure that you will not lose other’s changes on the same file). It is easy for someone to look at the ‘history’ of the file in source control and find who did what and when. So never attempt to cheat source control systems by changing the ‘Readonly’ attribute.