Most turrets rely on line of sight for targeting, so you can block them with smoke grenades.
Some details:

• Smoke grenades start emitting smoke ~2 seconds after throw, and cloud lasts for ~10 seconds. Throwing animation takes 1 second.
  
• Smoke grenades can be intercepted by turrets, in which case they'll create a smoke cloud, but it won't block anything on dedicated or official servers.. THIS ONE IS VERY IMPORTANT. It may confuse you, as bugged smoke cloud from intercepted grenade looks exactly the same as valid one, except grenade will float in the air where it was shot down.
  
• To avoid grenades being intercepted, throw them where turrets can't see them - either outside the turret's range, behind a cover, or within another valid smoke cloud. If turrets are set to shoot on players, you can turn around and throw them backwards - turrets will always prioritize closest viable target.
  
• If you're inside a smoke cloud, direct-fire turrets can't target you.
  
• If direct-fire turret is inside a smoke cloud, it can't target anything (you can attach C4 to a machinegun turret in smoke)
  
• Turrets regain their targeting immediately when smoke dissipates, so you'll need to throw new smoke grenade in advance.
  
• Works against Automatic Ballista, Automatic Turret, "Phalanx" Defense cannon and "Devastator" Auto Gun. Does NOT work against "Sparrow" AA system.
  
• Works against some traps (Arrow Wall, Flintlock Trap, Flamethrower)
  
• Vehicle smokes (both engine and grenades) don't have this effect..
  
• Smoke grenades placed on top of the vehicle do not move with vehicle.

Video demonstration:
https://youtu.be/3Hen2umvdrY

Video comparison of intercepted smoke grenades vs normal smoke grenades:
https://youtu.be/8E852_YygI8
Tested on 1.0.10 on dedicated server.
Retested on 1.0.11 on dedicated server.