Buy an air blower, like the Giottos Rocket Air. Face your camera's sensor towards the ground, give the blower bulb two quick squeezes, and give some time any dust to fall out. If you're lucky, those strands will fall out. If you're unlucky, you may have to bring it into a shop to be cleaned. You may also be able to have Pentax do something about it, but I'll let the more experienced people answer that.
Here are a few warnings: DO NOT USE COMPRESSED AIR! The chemicals used could corrode or ruin coatings and moving parts in your camera. Do NOT use your mouth to blow air into the sensor. Be careful if using lens wipes or anything involving applying something to the lens; if you have never cleaned a sensor with those wipes before, go to a shop and have a professional do it or show you how to do it.
In short, you yourself should only try a bulb blower. If that doesn't work, see a professional.