I would approach the ones who are the worst at getting back on time individually and quietly. Say that you are in a difficult position and don't like having to tell people off, but YOUR manager has told you to fine people! Say to them that it may only seem like a minute or 30 secs to them, but others are noticing and you can't make up your own rules re. breaks in this job. Ask them if the understand, and then let them get on with it. Say that they must consider that their breaks are now 13 mins long, not 15, so they get used to getting back on time. It may be petty, but all those minutes add up, business-wise!