I am getting a lot of spam lately, and its getting a bit of a job deleting each message in turn, then going to my "recently deleted" folder to permanently delete each one. I seem to remember there was a address you could forward spam to, to keep the message going round and round back to the sender.
That ain't gonna work. For one thing it's almost impossible to determine the sender. For another, in many cases, the sender is a bot running on one of the gazillion computers owned by people who brag "I've never used an antivirus program, and it's never caused me a problem".
You can gradually move your mail off the old email system by letting people know you new email account.
Better still have 2 or 3 email accounts for different things. Keep one for friends and family, another for web based activities, another for signing up for newsletters etc.
If you wish to delete a series of consecutive emails then click on the first one, hold down the Shift key and click on the last one. This will highlight all those in between then you can delete them all with just one click on delete. If you wish to delete emails which are not consecutive then again highlight the first one and while holding down the ctrl key highlight each one to be deleted. All highlighted ones will be deleted when you click delete.