How to delete Mac Mail from 1 device only

I have an early 2009 iMac running EL Capitan which is now obsolete and I've got myself a Mac Mini to replace it. I'm giving the old iMac to a young family member as it's still working fine for his needs, just not mine!

I can't do a factory reset or it'll put it back to early 2009 which is obviously no good so I'm manually deleting my files before handing it over to him.

My problem is with Mac Mail. I can delete all the messages but as I'm still using it on other devices I'm scared to delete the account as presumably that would delete it on all of them. Even if I delete all the emails I don't want him to be able to download the new ones.

Can anyone advise how I can best go about either deleting Mac Mail on the iMac entirely or alternatively stopping just the iMac from downloading new messages, but leaving my other devices unaffected, please?