The most effective way I know of controlling this plant, without the use of poisonous sprays which would also harm the primroses and other plants, is by cutting it down to ground level with a scythe or other cutting tool. The timing of this is very important. The most effective time to cut is towards the end of the plants flowering, but before the seeds have set. It is better to cut a little early rather than late, since the cut off plant is still capable of putting its last energies into ripening the seed and thus producing new plants the following autumn - if in doubt remove the cut tops and put them in the centre of a well made compost heap where the heat produced will destroy any seeds. Some of the plants might also put out short new growth from the base and try to flower and seed a few weeks later, so keep an eye out for this and cut them back again if need be. At least some of the plants will usually survive this treatment and come into growth again in the autumn, plus there will also be young plants that had not flowered in the current year that will try and flower in the following year, so it is important to cut again in the following year. By this time, populations of the plant will have greatly reduced, but there should still be survivors. At this stage I would suggest that you allow the reduced population to survive, but monitor them to make sure they do not get out of control. Although they can be aggressive growers, they do also have their beauty, bringing welcome green growth in the winter and providing a very valuable food source for pollinating insects.