Hello Marvin,

To remove invoice link , refer below mentioned solution.
Go to root/components/com_payplans/templates/default/_partials/default_partial_invoice.php, near line no. 44 find below mentioned line of code.

<?php echo PayplansHtml::link(XiRoute::_('index.php?option=com_payplans&view=invoice&task=display&invoice_key='.$invoice->getKey()), $invoice->getKey());?>

Replace with
<?php echo $invoice->getKey();?>

Let me know if you have any query.