There are three ways. Method 1 will disable source control for all projects. Method 2 will delete the link to a single repository for all projects. Method 3 will remove the link to a repository for a single project. I think Method 3 is the one you are looking for.
1: Through Source Control Management Preferences
In Xcode, choose Xcode-> Preferences, then select Source Control and uncheck Enable Source Control option.
2: Go to Xcode->Preference->Accounts->Repositories and delete them.
3: You can do it manually this way: Show Hidden Files on your Mac and delete all the .svn or .git or .tf files (according to the source control) folders from the project.
Example svn directory:
