The issue does not lie within the JOpenSim Register itself, but rather requires an override of the Joomla core in the user registration component. To do this, navigate to Joomla admin, then Extensions, and Template. Choose your template and proceed to /html/com_users/registration/default.php. I resolved the problem by adding the following code:
?>
<script type="text/javascript">
document.addEventListener('DOMContentLoaded', function() {
var input = document.getElementById('jform_jopensimregister_jopensim_firstname');
input.addEventListener('input', function() {
this.value = this.value.replace(/[^a-zA-Z0-9]/g, '');
});
input.addEventListener('paste', function(e) {
e.preventDefault();
var text = (e.originalEvent || e).clipboardData.getData('text/plain');
this.value = text. Replace(/[^a-zA-Z0-9]/g, '');
});
});
</script>
<?php
P.S. use Helix Ultimate Framework