Use acetone (nail polish remover) and an old toothbrush, reapplying the acetone a number of times and brushing to remove it layer-by-layer. Work on the outside and then on inside. May not come off entirely. Good luck!
could be tricky, but depending on the value of the said trousers a really sharp pair of scissors usually does the trick, then all you need is a good 'invisble repairer !'