Spayed cat still goes into heat

My Bengal was spayed over a year ago yet went into heat a few months ago. Blood test concluded she was still producing hormones. The vet said there is likely a "stub" or loose "cell" still producing hormones and he did exploritory surgery about 4 weeks ago to cut away more of the folopian tubes and some soft tissue in hopes of eliminating the cause. But said he it is possible they may never find it. 24 hours after her surgery she went back into heat and has continued to go in and out of heat ever since, spraying and howling non stop. The vet said it could take a month for the hormones to leave her system so to be patient. It has now been one month and no change. I am now concerned the surgery has failed. I am wondering what options are available if she continues to produce hormones and go into heat? HELP!