How can I prevent this from happening

A Django TextField is rendered as a HTML textarea.
Looking at this question, you could use style="resize: none;".
If you would like to add that in your views/form (and not in the templates), you could try something like:
class MyModelForm(forms.ModelForm):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.fields['my_field_name'].widget.attrs['style'] = 'resize: none;'
or if you have a form instance
form.fields['my_field_name'].widget.attrs['style'] = 'resize: none;'
