Do not block the user from completing the form. Allow them to complete the form and fix the errors after.
A study in 2007, titled Usable error message presentation in the World Wide Web: Do not show errors right away, was done by Javier A. Bargas-Avilaa, Glenn Oberholzerb, Peter Schmutza, Marco de Vitoa, and Klaus Opwisa.
Online form validation can be performed in several ways. This article discusses two empirical studies with 77 and 90 participants, which have found evidence that the best way of presenting error messages is to provide the erroneous fields after users have completed the whole form. Immediate error feedback recommended by the International Organization for Standardization (ISO) showed the worst performance in these studies. Where presented with immediate feedback, users often simply ignored the messages on the screen and continued completing the form as if nothing happened. These results lead to the postulation of the “Modal Theory of Form Completion”: Users are in either “Completion” or “Revision Mode” when filling out online forms. These modes affect the users’ way of interaction with the system: During Completion Mode the users’ disposition to correct mistakes is reduced, therefore error messages are often ignored.
In short, users tend to have two different modes when completing a form: completion mode and revision mode. They will complete the form first and then fix errors, even if errors are presented in real time inline with the form fields.
The study found that users tend to ignore immediate error messages when they are in completion mode. It found that the most productive way of returning errors was embedded in the form all at once, after the user had completed the form and attempted to submit.
Punishing the user (i.e., blocking them from continuing) is not appropriate when they've made a simple mistake. However, based on your specific work flow and needs, it could be appropriate to display an inline message that weight limits are outside a required range -- but allow the user to ignore this and continue if they choose. When the submit the form, they will address the error when it fails to submit.