There's a difference between being straightforward, which is what this situation sounds like, and tactless, which would be saying something like, "Stop swearing in front of the kid, you immature, moronic potty mouth!" The comment, "Enough with the swearing already," is already quite tactful.
Tact is more of a concern when offering a critique or criticism...the difference between constructive and destructive criticism. For example, when someone asks you why they didn't get promoted, or why you didn't assign them to a particular project, sitting down and explaining the areas they need to build up more strength in before getting such a promotion or assignment is tactful; telling them they screw up everything they touch and you can't trust them to do anything if you're not standing over their shoulder the entire time is tactless.
Some people will always have a problem with the straightforward requests or instructions or even demands, because the more straightforward it is, the less room they have for argument to do things their way instead of your way. In some situations, it's better to consider alternative approaches and discuss things before making a request, but in others, such as the example provided, where there is no other acceptable option from your perspective, then there's no point beating around the bush or leaving ambiguity in the request.