No really... You cannot.
What happens is when pressing the backwards button after a smile the cursor, the cursor will actually delete one character before the smile then jump over the smile to delete all the characters before the smilies. It does not matter how many times you reposition the cursor after the smile or a series of smilies. The smile will not delete.
Note: I have not tested other mobile browsers or other mobile Operating Systems combinations. If others could test their mobile browsers to confirm. It would be much appreciated.