To me, overactive muscle means that it tends to dominate the movement. It also usually means the antagonists are not as efficient as they should be.
Quite often, an over active muscle also displays a lack of proper flexibility. However, this isn't always the case. You're a prime example.
In addition to SMR and stretching (which I'm sure you're already doing), also strengthen the glutes as alexus stated.
Also, when performing movements in which you want to minimize hip flexor engagement, try stretching them for at least 30 seconds. The stretch should never cause pain but should be aggressive enough to stimulate the golgi tendo organ. If held for at least 30 seconds, the GTO should kick in and, through a process called autogenic inhibition will create an inhibitory response (make them less likely to fire unnecessarily). While doing these stretches, engage the antagonists (the glutes) and, through a process called reciprocal inhibition, will aid in the stretch.
The key is at least 30 seconds to activate the GTO prior to performing a movement in which over activity of the hip flexors is undesired.
Obviously, if you want to statically stretch a muscle in between sets (an old school body building technique that works for many), you will want the duration of the hold closer to 10 seconds to ensure the GTOs are NOT activated and your strength in the lift is not compromised.