Thorough preparation can be lengthy and not very exciting - but it pays off. It ensures smooth and efficient ongoing work. When all the definitions, settings, and initial data are made in advance, work flies! But even perfect prep work should be refreshed from time to time - new definitions should be made and existing ones may become irrelevant for short or long term.
This is why SAP Business One enables you to set irrelevant UoMs included in UoM groups as inactive:
By default, all UoMs within a group are set to "Active". When deselecting the checkbox, the given UoM becomes inactive, and will not appear in any dropdown list of Unit of Measures, and will be hidden in Item Master Data respective sub-windows - i.e. Bar Codes sub-window, Purchasing tab > Purchasing UoM Code, and Sales tab > Sales UoM Code.