|The one person who definitely knows if you are flirting is you. Deep down, the flirt always knows. If you are flirting with the customers, it's also worth noting that you are flirting with trouble. But, if you can honestly say that you are not a flirt, that's the end of this issue.|
In terms of dealing with the mistaken image of you, there is not much direct action to take. If it makes you feel better, you can approach the gaggle of co-workers and discuss the matter. However, don't expect them to suddenly change their opinion. The problem is that you can look at your behavior and see an animated, personal, and enthusiastic approach to your customers, while the gaggle looks at the exact same behavior and sees an advanced stage of amorous banking.
The reasons for their determination that you are flirting are probably based on some wishful thinking. For example, they may be jealous of your friendly style of dealing with the customers and wish that they could do the same. Since they cannot, they redefine it as flirting. Perhaps they have actually done some flirting in the past, or secretly wish that they could be doing so now, but that's not in the cards. This draws them to the issue of flirting and causes them to see it in cases where it does not exist.
The fact is that the comments about your supposed flirting did not come from your manager, and there obviously has not been a groundswell of complaints from the customers. Just because some fellow employees have gone petty does not mean that you need to change your behavior.
Regardless of your actions, the little group would keep the flirt label pinned on you because of their own baggage. If you are not a flirt, don't focus on their image of you. It's their problem...not yours. If you want to build better work relationships, focus on some of your other co-workers who do not share the flirting fantasy.