Best for you to create a new reseller, move the users to the new reseller then only remove the old reseller so that the accounts under the old reseller will not be affected.
However as far as I know or remember, it should not affect the accounts under the old reseller which you are removing. But best to move the accounts to a new reseller first then only remove the previous reseller. That way you are 100% sure it will not have any problems with the users.
Why would you want to remove the reseller account? I'm not sure if all accounts stay present when removing a reseller.
But you could also just change the passwords so the original reseller can't use it anymore.
Or you can also change the resellers username like this: cd /usr/local/directadmin/scripts
./change_username.sh olduser newuser
Be aware that this effects all database which are in use by the reseller at user level (so his own databases) and you have to change the login credentials to the database in the application. If he has something like this present.
Or indeed as aodat2 says, move the accounts to admin or another reseller account is also possible.
The answer to this question is yes based on this GUI:
When I delete this reseller, it asked for confirmation that it will also remove the other users under this reseller. So if you don't want to lose other users under this reseller follow advice from @aodat2 and @Richard G