In Infigo Designer, variables do not have native support for being fixed values. However, if you have advanced version enabled, then you will be able to add a property to the variable to set it to read-only, disallowing the user from editing the variable.
Whilst this is not a configuration which you will always need, this can be very useful in certain situations, such as when you are trying to use variable suppression but you want certain information to remain present in the document.
In the screenshot below, I have enabled advanced version and set up a basic field that contains two variables: one which will be fixed, and one which we will allow the user to edit. In order to add the property that can make the variable read-only, you will need to select the variable you wish to make fixed and click on the 'Add...' button highlighted in red.
In the menu that appears, you will need to set the name to 'readonly' (case sensitive), and set the Type to 'Boolean' - this will make it a 'true or false' variable.
Once done, simply click 'OK', then save the PDF. If you would like to make this available for all text variables in the future, then you can do so by enabling 'Create as predefined property' in the above step. Please note that you will need to have opened Acrobat as an administrator prior to doing this, else your operating system will prevent Infigo Designer from saving this change.
Now that the variable has been created, all it takes is to set the readonly property to 'True'. This can be done either by selecting 'True' from the dropdown, or double-clicking 'False', which will change the value from False to True.