Can a laser pointer ruin a long exposure photo?

Can laser pointers be used?

Yes and no. If you are attending a Star Party hosted for astronomers and astrophotographers, then laser pointers are usually among the items listed that you may not use. Some star parties hosted for astronomers (not the public), may schedule an event when a tour of the night sky (via laser pointer) is performed and all astrophotographers know this and aware that they may not want to be imaging during that particular session. Laser pointers are not used or permitted at any other time.

If the star party is being held for the public (not for astrophotographers) then laser pointers are often used during sessions on each night and astrophotographers know that this. They might be used at any time and without warning.

Will they ruin images?

This is a "it depends" answer. Laser pointers used by amateur astronomers are green (typically 523nm) lasers. This particular wavelength is used to because human eyes are very sensitive to it (it is smack in the middle of our visual spectrum) and it reflects off the particles in the air showing an obvious beam. Other colors can be used... but unless the laser is especially powerful, the beam isn't visible.

Professional astronomers will use lasers to create an artificial star for purposes of adaptive optics. These lasers excite gasses in the sodium layer (about 80-100km above sea level) and since the gas is sodium, they use a sodium laser to excite these gasses to create the artificial star. Sodium lasers are yellow. These are especially powerful lasers. Meanwhile, the lasers amateur astronomers use are typically not very strong. Though laws & regulations may vary by country or local government rules, these tend to be fairly low power lasers. Here in the United States they are typically 5 milliwatts or less. It turns out your ability to see these lasers really depends on how close you are to the source.

A 5mw laser is easily visible to people just standing a few feet away, but people a bit farther away will struggle to see it. People on the far side of a field wont see it at all.

When I do astronomy outreach, I typically use a 5mw laser and those standing near me can see the beam. If I were to do a tour of the night sky using a laser for a moderately larger crowd (say... 25-50 people) then I'd need a stronger laser ... maybe 50mw ... otherwise those farther away will struggle to see the beam.

If you're out with a few friends and worried you might be ruining images for someone taking images 10 blocks away... no worries. Their cameras will not detect the laser beam.

Safety

As always, use proper care when operating a laser and avoid pointing them at people or aircraft. This keeps the local constabulary from having to deal with the situation.