Thanks janus_zonstraal, I will be trying that for sure
david63, I thought of the timeout too, and thought a htaccess would help there as I though it would overcome the possible limit by php.ini of the host. As it is shared host, I suppose they cannot change it for one user. I was a bit hoping there would be a tool for phpBB that would do the same task but in multiple turns, thus overcoming the timeout problem.
kevC, thanks for the tip. I have previously used that (as a tip for others, I installed a plugin for my browser to select all checkboxes, as phphBB does not have that option), but was kind of hoping there would be already a tool for that, i.e. to delete all posts of certain user/s. But I guess this is the only way currently.