If you don’t wish to bother with undoing guy wire tie points to re-tighten them, or, maybe there is no more twist left in spiral twisted guy wires give this method a try:
Form a piece of 3 mm aluminum wire shaped like this.
Slip the guy wire between the handle bars.
Now turn the handle bars wrapping the guy wire as if on a spool.
The guy wire will remain wrapped in place when it is set (locked) against the rounded end of the re-tightening widget.