Hi, and Welcome to Mr Excel
You could insert a userform (with the checkbox on it, along with an OK button, and some text instructing the user to accept your terms by checking the box) into the VBA section of your project, then insert a line into the _Open event of the workbook to show the userform modally.
Once the user clicks the OK button, if the checkbox is checked the workbook continues, if it's not then use the......
....line to close the book.