With thistle the key to control is vigilance and persistence. Do not let the plant flower and go to seed. Mowing/clipping the tops whenever they appear will also help starve the roots. Use a small paint brush to apply a non-selective systemic herbicide, (active ingredient glyphosate as in Roundup and others) to the cut ends of the top growth will further help to suppress/kill the plants. Follow label directions for timing of the herbicide. Please see the attached publication from the Plant Conservation Alliance for management options
http://www.nps.gov/plants/alien/fact/ciar1.htm 
mh