We often put a 2X4 or 2X3 down and screw it to the joists between the decking if there is enough space, so it doesn't make a hole in the decking. It also allows you to move your ladder around and still be secure. We also use the putty knife trick on occasion if the spaces are tight, but I prefer the more secure 2X method. My first year in business (almost 30 years ago now), one of my painters had a ladder set up on a deck and it kicked out. He was in front of a sliding door so when it hit the back railing, it shot him through the glass of the door showering stain all over their kitchen. Because of that, I am a stickler about securing ladders especially on decks, sidewalks, or any where there is any potential to kick out.