For Sourcetree users: If you want to ignore a specific folder, just select a file from this folder, right-click on it and do “Ignore…”. You will have a pop-up menu where you can ignore “Ignore everything beneath: <YOUR UNWANTED FOLDER>”
If you have the “Ignore” option greyed out, you have to select the “Stop Tracking” option. After that the file will be added to Staged files with a minus sign on red background icon and the file’s icon in Unstaged files list will change to a question sign on a violet background. Now in Unstaged files list, the “Ignore” option is enabled again. Just do as described above.