IRRC In phpBB it uses short codes to "link" the php script.
You can simply remove it and it will stop working, But test this first and "make a backup" Just copy paste in the same folder and then use the copy if all goes wrong. (just rename it)
If that fails The php Script for the login will at some stage have something like
echo,
- user login js
Just place // before the js script <script> or echo and it will stop initiating.
The phpbb website will have a better grasp of this.