Hi David thanks for your reply.
I guess I'm just having too much coffee and lack of good sleep.
Here is my problem came from:
I used the level dimension without the symbol, just a clean text with frame.
Then, after I put them on the slab, I moved them around to have a better layout. Without any suspicions, I just dragged and dropped them by holding on the beginning of the text node. I thought that was the hotspot of the symbol.
And that was how I used the node to select it and tried to delete them.
Nothing is happened, because it is not the correct node.
The solution is to revert the position of the symbol, by selecting the text, and go to the info box, look for the revert position button. Then select the correct node which lies, hidden in the centre of the dimension text. YES.