How 'bout right clicking an email and selecting black list this sender or ip or?, can I do that?
Yes. The interface also places check boxes next to each message. One for 'delete' and one for 'bounce' which adds the email add'y to the MW blacklist and then bounces the email back to the sender/spammer so that it looks to them as if it were never delivered. This will pare down the amount of spam over time as automated systems will remove your address after it keeps receiving non-deliverables.
Can I remove someone from a blacklist the same way?
Yes. I'm not sure about add/remove using right click. I never tried it but the blacklist is easily accesible for modification.
Or do I have to stop what I'm doing and open the mailwasher interface and add an ip or domain or ? to it's list, then save and close, and then close and re-open my email client?
Once you open the email client, it will download the messages on the server to your local machine and will no longer be viewable by MW. Unless you do not have that as the OE default, but the same thing will happen when you click Send/Receive in OE...unless you have selected to "Leave Messages on Server" within OE options.
You should do all such admin within MW before opening OE.