What is the best way to reprimand my dog?

Shes a cairns cross (like Toto on acid). I noticed lately shes misbehaving just to get my attention when we're out walking. I have to take the blame for her current behaviour because at first I could never get her to come back to me, but I discovered if I rewarded her with treats she got the message and she comes running back. But now shes barking at other dogs (always ones at least 3 times her size), men on bikes and men running, she does this when she is on or off her lead. When she stops barking she looks at me like shes done something great and expects a treat. I try to be stern and look her in the eye, and tell her no, but shes not getting the message. Any hints about what to do next. thanks