In tortoiseSVN when I'm on commit looking into file changes, is it somehow possible to check the changed line of block of lines not to be commited without actually deleting it from the local file?
The closest thing you have for this is menu command named "Restore after commit". This command will save the file as it was when you issued the command. Then you can change the file and remove the previously added line (so you manually revert any changes that you don't want committed in the next commit). Then you commit. After the commit Tortoise will overwrite the file with what it had saved, and in effect restore the line.