how can I limit the access for bluetooth units

I have a pair of headphones and a handfree in my car, but if I turn on bluetooth, then both iPhone and iPad will constantly be searching for new bluetooth devices. I would think that it will use a conciderable part of batttery time and leave my device open for others. I have a vague memory of having had an option to "limit connection to already connected units". Does ios have that and where and how do I find it?