Are you using equipment other than the reformer, for instance, are they doing mat work? Exercises on the chair, for instance, Pull Ups, would certainly encourage these strong male clients to activate their Powerhouse. That's the beauty of the system. If you can't get what you want in one area, you can in another.
If you don't have access to, or are not trained to use other equipment, rely on biomechanics and understand that change happens over time. With only five or six lessons habitual patterns won't change but the awareness of those patterns will be brought to the surface. If the hip flexors are strong and over-working focus on the hamstring, glutei and adductor activation in order to balance the strength of the hip flexors. Work to align the pelvis and strengthen the core. This will begin to allow the hip flexors to lengthen. A ball between the thigh is good but try doing reformer foot work on light springs while cueing them to pull the carriage in with their hamstrings and bottom as one technique to activate the posterior. Ask them to press the carriage out from the bottom and Powerhouse and cue oppositional length for the front of the body.
Recently I worked with a muscular, tight hip flexor personal trainer gal and this worked magic for her. She reported that her hamstrings and lower lower abdominals were really sore and that she felt stretched out through the front of her body. Our next session proved to be really good because she had worked on getting that feeling in all her other exercises that week. Her hips are still tight but she is now aware of how to work the opposite side in order to spread the load and work more correctly. This is called reciprocal innervation and you can do it too if you think through it. Good luck and have patience and be persistence. Don't chew things up too much with these guy clients either, get them moving and soon they will feel the beautiful benefits of Pilates.