How +do +I +stop +plus +signs +being +added +between +every +word?

First check the System/Keyboard/Shortcuts preference pane to see if somehow a shortcut has been created adding the + in lieu of a space.

Then give this a try: boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

NOTE: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

That sounds like a keyboard issue.

1. test another keyboard with your iMac.
2. test your keyboard with another Mac or PC.

Most any Apple, PC or Generic USB keyboard will work with both Mac’s and PC’s.