If you prune it, you will be cutting the plumes off and that is the pretty part of the plant. If it has spread too much, I would take a sharpshooter and decrease the size. I have heard that some gardener’s purple fountain grass comes back but it never has for me. If it is in a pot and kept in a greenhouse in the winter, I suppose you could cut it back in the winter and it would remake plumes in the summer when it is moved outside.
Purple fountain grass is an annual grass in Parker County but sometimes pennisetum is known as fountain grass so make sure you have the true purple fountain grass and not some other ornamental grass. If you have perennial, ornamental grasses like muhly, miscanthus or pennisetum, they should be pruned in February, right before new spring growth BUT if mine gets raggedy from lack of moisture, I prune them anytime. I will prune the dead stems out of a plant during the extreme hot weather.