You can use Turpentine to remove the paint, its the least harsh of the thinners and shouldn't damage the ABS.
Dab over the stripes with a cloth soaked with the turns so it starts to soften the paint then see if it will start to wipe away. It may need a bit of coaxing so you can also use a wooden toothpick to lightly scrape it off the surface.
Stencils are available through Trooperbay in a hand painted style and are the easiest way to have them re applied.
A few tip when attempting to apply the stencils and repaint:
Spray both the helmet area and the sticky back of the stencils with water that has a couple of drops of dish washing liquid mixed in.
This will allow you to move the stencils easily into the correct position. You want approx a pencil with gap between the cheek and the top of the tube stripes.
When happy with the positioning use paper towel and soak up the soapy water also make sure to squeeze and of the liquid out from under the stencil and make sure they are thoroughly dry.
When dry, use white paint first to paint through the stencil lines. This will certainly reduce the chances of the blue paint running under any micro gaps of the stencil. Trust me it makes life much easier for any potential clean ups.
If you do get some runs, simply dip a toothpick in Turps and lightly scratch away the runs.
Hope this helps.