How to jam motion sensor without triggering the alarm?

I wrote the security technical advisor to the movie Sneakers, Mr John Strauchs about how to jam motion sensor. Sneakers includes a scene where intruders increase the ambient temperature of the room in order to defeat the IR motion sensors. John wrote the technical details of that scene (including the purposeful errors).John is a former operations officer at the CIA, and designs security systems for major locations around the world. This is a synopsis of what he had to say. There are a couple factors to consider in your IR laser scenario.

First, is the complexity of the sensor itself. Top-of-the-line sensor units are a cluster of multiple sensors (up to 16) that create different zones to monitor. If you want to blind this type of unit, you would need to blind all 16 sensors. Cheap sensor units have only one sensor, and manufacturers break up the field into zones using physical means. One need only blind the one sensor in a cheap model.

Second, is the nature of the IR laser itself. It would have to mimic the IR signature of the thermal energy in the sensor zone it was in when you first turned it on. The IR signature would have to match everything between the sensor and the barrier in that zone. You could obtain this with an IR camera and some fancy math to program the laser, but it would be tricky, if not impossible, assuming you could get the IR laser to even produce a signature something close to the thermal equivalent to room temperature. To have this level of control this would require a near-weaponized level of complexity in an IR laser. In short, using a laser would be the easiest way to trip the sensor, not blind it.

John says that this is unnecessary, though, if you have close physical access to the sensor when it was off. A simple silicone spray will blind the sensor, and is invisible to the naked eye. It leaves a trace, but it might not be discovered until much later, if at all. Alternatively, you could place glass in front of the sensor when it was off, which would do the same thing. Glass and silicone sprays block the IR wavelength bodies emit.

That last piece of advice is the kicker for defenders. PIR sensors need to be inspected regularly, and even opened to see if glass or plastic was inserted inside the unit. Sometimes contractors will "tune" the unit by putting plastic inside to make the unit less sensitive. Always test PIR units by doing boundary tests, not by waving your hand right in front of it.

TL;DR: It depends on the sensor manufacturer, and you would need a weaponized IR laser to even attempt this. Inspect your PIRs frequently for glass, plastic, and coatings, and test frequently using boundary tests, not waving your arms in front of it. You get what you pay for in sensor units.