I have accumulated over 300 emails. I have tried to delete them unsuccessfully. by ticking the box on left, then going to right hand side and clicking on delete. They go but then reappear after sometime. I would prefer to delete them at ,say, 50 at a time if possible. I also need to know how to block some messages. Grateful for help - this is so frustrating.
Going by your description of how you are trying to delete your emails I have presumed that you are using webmail on a browser.
Amongst the many other problems with the email webpage UI (User Interface) there also seems to be a problem with deleting emails in bulk. BT are apparently aware of the problem and are allegedly working on a fix.
The following are not suggested as "fixes" but as a "work around" you could try the following:-
Click on the box to the left of the first email you want to delete and assuming you want to delete all the emails that are below that you can either press on your keyboard "shift" key and "PgDn" key repeatedly and it will mark all the other emails with a tick or use the "shift" key and the down arrow. This will also mark all the boxes with a tick or tick the first box you want to delete and holding the shift key move to the last email you want to delete and tick its box and release the shift key. All the emails in between should now be ticked.
If you don't all the emails that have been marked with a tick deleted you need to go down the list and untick the ones you want to keep.
Once you have done that you then click on the "Delete" button in the menu bar which should delete them on mass however if it does not work, once they have all been ticked, you should try to drag and drop them into the trash folder.
The same applies if you want to move emails to another folder, use drag and drop.
As regards blocking messages, you can block the sender of email by going to the settings which are found by clicking on your username at the right hand side then go to "Mail" > "Block sender" and adding their email address or if you want you can block all emails from the domain of that the sender by using suffix of the senders email address ie. *@example.com. Note the use of an asterisk.