Yes, using a technique called Fade Extension.
1. How to do it.
There should be at least one high-level Techforcer in the group. This person is going to be the damage tank, so they should have ICTV Basic and Bulletproof Aced. To accomplish this, they'll need to be at least level 82.
Around the middle of the first (lowest intensity) assault, the tank should find an area with good cover away from the main mission. The roof of the Big Bank or the basement of Rats Day 1 are perfect for this.
As the assault slows down, the tank should kill every nearby enemy but one.
The tank then sets up a cycle of alternately allowing that one enemy to shoot most of their armor away, and then ducking behind cover to regenerate.
Meanwhile, the other three players take out all the remaining enemies.
There will be no indication that you've gotten this right except that the assault indicator will continue to say that there is an assault in progress, even though no new assault waves will spawn.
6(a). If the host has HoxHud, they will be able to see that the assault is in fade. Thanks Compro01!
You're done! As long as one player continues to tank damage, the assault will never end and no new assault waves will spawn (but scripted spawns will still happen), leaving the rest of the crew free to cook meth or do whatever they like.
2. Why it works
Assault waves are broken up into segments, the most important of which is called "fade". Fade is located at the end of an assault. During this phase, only scripted spawns are allowed - no new assault waves will spawn.
Fade ends either when less than 7 law enforcers are alive or after 355 seconds. However, there is also a 30 second damage countdown. Fade does not end ever as long as players have taken enough damage during the last 30 seconds.
Therefore, if one player is tanky enough, they can prolong the assault forever, even though no new cops will spawn.
The Long Guide, by Frankelstner