How do you catch a runaway blimp?
Historically, a runaway airship cannot be caught or entered. Eventually it will come down - they all did so far. Only in one case did a Zeppelin fragment of a ship which broke up in a storm make a soft landing so the crew aboard could survive. In all other cases both ship and crew perished or were never seen again.

While the Zeppelins had vertical shafts which allowed the crew to climb to the top of the hull, a blimp can only be entered below the hull. Trying to lower someone James-Bond-style from above will fail, because the big hull will shield the entry points of the gondola very effectively.



Defensive gunner position on a Zeppelin. Note the square hatch at the bottom which leads down to the bottom of the ship.

Also, shooting it down will be an exercise in futility. All it will do is to increase the rate of helium loss, so the flight will be some hours shorter. Helium is inert and will not burn, and those small holes will be tiny relative to the big surface of the hull.

At 15,000 ft it will be hard to try to catch one of the lines most likely hanging from the blimp with a helicopter. Remember that the blimp floats, so the helicopter has to hover at this altitude. Very few are able to do so. Towing will only be possible with another blimp or a helicopter; an airplane will not be able to fly fast enough with a blimp in tow.

The next trick depends on the color of the hull: If it is dark, covering it with something highly reflective will cool it and reduce the blimp's buoyancy. I have, however, no idea how to get a big blanket on that thing. The same will happen when night falls, but first the air will cool and the blimp, which will cool down more slowly, will go up. Later in the night and early in the morning it will sink again.

The wet parts of rain clouds will mostly be below 15,000 ft (water content is proportional to air density, after all), so a shower which drenches the hull and makes it heavier is highly unlikely. There is no obvious way how this thing will come down fast. Except, that is, it climbs further and the hull bursts. This will change the dynamics of this escape in a dramatic way.

The chase planes will help ATC to route traffic around the blimp, and all that can be done is to watch it. With luck the landing will be soft enough that it is not a complete write-off.