I have a nintex form with a checkbox. If a user selects the checkbox, it will populate two fields with the user's name and the date the box was checked. This data is saved to a sharepoint list.
Once the box is checked and the form is saved, it will not accept any updates if the box is checked a second time. The original input remains but all other inputs still work like text boxes. Would a radio button be more appropriate or it the flow in Sharepoint causing the issue?
I've been working with Nintex Workflow and Forms for over a decade and I am a Nintex Certified Process Automation Expert. It is possible that the second half of this answer may turn out to be the answer you're looking for.
Concerning the original question
First, are you using On Premise Nintex forms or O/M365? If on premise, what version are SharePoint and Nintex W/F are you using? This can make a significant difference.
Second, could you perhaps post a screenshot of your forms demonstrating this behaviour? Depending on the following though you may decide to not bother with screenshots and change the column type.
Third, I'm curious as to what your forms settings and the settings for the field are. With on premise forms there are options to configure forms in view or edit mode. Have you checked the forms settings? Have you checked how you have the checkbox configured?
Concerning a more direct answer
Fourth, concerning what would be appropriate. A checkbox works well for a boolean choice (True/False). However, I tend to avoid using checkboxes in SharePoint and on Nintex forms unless it really makes sense to do so (which is rarely).
When it comes to designing lists and forms, I'll typically start with a radio button in the Nintex Form with an option such as Yes, No. I find that this is not only more intuitive for the end user, but also for me when I am working with workflow actions such as Set a Condition in the workflow designer. In UX testing over the years I have found that most people find a Yes/No radio selection always clearer than a checkbox.
Whether I set a default for a radio button depends on the process analysis and determining if forcing the user to pay attention to the field is the priority. If a default is not used and a selection is required then a rule will need to be setup for the field ensuring that it is not null or empty. When the choice is not one demanding attention for a process activity I'll set a default reflecting the most likely branch. This allows for few clicks on the form, and makes sense a lot of the time. However, process analysis is critical when it comes to UX design in my opinion.
As I read your question, I have the feeling that you're experiencing one of the reasons why I typically go with a radio choice field in SharePoint, set the choices as Yes, No. True of False can be used, but I find those words less intuitive than Yes or No. Human beings related more to yes and no.