How can I make a Django form field contain only alphanumeric characters

You would use a validator to limit what the field accepts. A RegexValidator would do the trick here:

from django.core.validators import RegexValidator

alphanumeric = RegexValidator(r'^[0-9a-zA-Z]*$', 'Only alphanumeric characters are allowed.')

name = models.CharField(max_length=50, blank=True, null=True, validators=[alphanumeric])
email = models.EmailField(max_length=50, unique=True, validators=[alphanumeric])

Note that there already is a validate_email validator that’ll validate email addresses for you; the alphanumeric validator above will not allow for valid email addresses.