If the ABS plastic burns inside the Plastruder, it can create an oxidized ABS plastic byproduct that will occlude the Plastruder print head. This is difficult to repair, and can be caused by setting the temperature of your Plastruder too high.
The ideal temperature for melting ABS plastic is around 220 degrees-C.
To avoid this condition:
Every time you remove the ceramic tape, visually inspect your nozzle and make sure the thermistor is well placed and not peeling off.
Validate that your thermocouple works. You should see the thermocouple reading change soon after turning on the heating element. Be suspicious if your thermocouple reading does not change.
Don't set the target temperature for the Plastruder higher than 225.
Hypothetically, the ABS plastic could get scorched if the plastic filament feed gets stuck, and the same chunk of plastic heats in the print head for too long a time. In practice, however, we've cooked ABS plastic in the print head for extended periods (10-20 minutes) without adverse effect. But: don't run this risk unnecessarily.
This can be fixed by totally disassembling the heater barrel and nozzle and cleaning out all plastic and other debris from the nozzle and barrel.
And remember that if your repair doesn't go well, you can always buy separate replacement parts from the Makerbot store.
Gentle manual cleaning
If you are lucky, You can get a large amount of the plastic out of the barrel using the heater itself. Carefully unmount the extruder head from the drive assembly and then remove the PTFE insulator from the barrel. Place the whole assembly on a heat resistant surface and connect it back to the plastruder controller. Carefully bring it up to temperature to melt the plastic and then gently push a rod down the barrel (a 3mm bamboo stick for instance), manually extruding the contents of the barrel out. This might be easier if you hold the assembly by the retainer washer with a pliers or something.
Note that if this is easy, you probably didn't have a blockage and may not have needed to do this. If it is difficult, you may have a blockage. If this is very difficult, this method might not get you anywhere at all.
If this doesn't seem to be getting anywhere, try removing the nozzle from the barrel. First, remove the thermistor to prevent it from being damaged. Then put two M6 nuts on the barrel, tighten them together so they grip the barrel well. Heat the barrel up carefully and use an M10 wrench to hold the nuts, and an M8 wrench to hold the nozzle, and carefully unscrew the nozzle. This will become easier as the nozzle approaches the plastic's melting point, but note that the wrench will cool the nozzle while they are in contact.
Dissolving the plastic chemically
If you use this method, you almost certainly will need to totally remove and later replace your heating element and rebuild the entire extrusion head. Also note that most solvents are highly flamable, and you should only start this after the parts are all cool.
Acetone and several other solvents dissolve ABS, but very slowly and only with some physical help. This may take several hours to a day with frequent agitation and prodding. It helps if you can get most of the plastic out via a manual method first.
More violent methods
Other more violent methods (including drilling and blow torches) are known to work but should be taken at your own risk.
If the threads of the nozzle are clogged with plastic, use an M6 thread tap to make the threads perfect again. This works quite well and, as long as you're careful, it should be very safe for your nozzle.
If the barrel is clogged with plastic, drilling works well to get the plastic out. Just make sure the drill is spinning fast enough at the beginning to make sure the drill bit doesn't get stuck. Also, as long as you are careful with this method and TAKE THE NOZZLE OFF FIRST you should have no problems. You may have to heat the assembly up first to get the nozzle off.
Using a blowtorch to clean the nozzle is dangerous. as the point of it is to burn all the plastic out of the nozzle. This should not be done inside, and even if done outside, you should avoid breathing the toxic smoke.