You can Simply use this xml snippet for this to disable auto option

<?xml version="1.0" encoding="UTF-8"?>

<templates xml:space="preserve">

    <!-- Inherit the existing template for NameAndSignature -->

    <t t-inherit="web.NameAndSignature" t-inherit-mode="extension">

        <xpath expr="//div[@t-if='state.showSignatureArea']" position="attributes">

            <attribute name="t-if">true</attribute>

        </xpath>

        <xpath expr="//canvas[@t-if='state.showSignatureArea']" position="attributes">

            <attribute name="t-if">true</attribute>

        </xpath>

        <!-- If wanted to hide Auto button -->

        <xpath expr="//a[@t-on-click.prevent='onClickSignAuto']" position="replace"/>

    </t>

</templates>

and add this xml file web.assets_frontend. along with this js file in static

/** @odoo-module **/

import { NameAndSignature } from "@web/core/signature/name_and_signature";

NameAndSignature.prototype.onInputSignName = function (ev) {

    // Always force name to a string

    this.props.signature.name = ev.target.value || "";

    // DO NOT change showSignatureArea state here, so area stays as you want

    // DO NOT auto redraw signature

};

NameAndSignature.prototype.getCleanedName = function () {

    const text = this.props.signature.name || "";

    if (this.props.signatureType === "initial" && text.length > 0) {

        return (

            text

                .split(" ")

                .map(w => w[0])

                .join(".") + "."

        );

    }

    return text;

};

I've found a better solution which works generally and not just for the portal:

odoo.define('your_module.name_and_signature_extension', function (require) {
"use strict";

var NameAndSignature = require('web.name_and_signature').NameAndSignature;

NameAndSignature.include({
    init: function (parent, options) {
        // Ensure options is an object to avoid errors
        options = options || {};
        // Set default mode to 'draw' if not provided
        options.mode = options.mode || 'draw';
        // Call the original init function with the modified options
        this._super(parent, options);
    },
});

});

put this snippet in a seperate modul at static/src/js 

and load file in web.assets_frontend section in manifest

"assets": {
	​"web.assets_frontend": [
    	​"/your_module/static/src/js/name_and_signature.js",
	​],
}

with this code the default_mode is set to 'draw'

I found a solution:

Edit this file

portal/static/src/js/portal_signature.js

change in line 161:

'mode': this.$el.data('mode'),

by

'mode': 'draw',

and add this later to the same file:

var style = jQuery('');
jQuery('html > head').append(style);