I know that getting rid of the trial can be done in one of the tabs/categories, but I can't for the life of me remember which one and I'm not on a machine where Malwarebytes is installed.
I have not ever, at least recently, been able to install the free version without getting the premium trial, but I immediately dig through those settings tabs/categories and uncheck the checkbox (at least I think it's a checkbox) to cause the trial to end immediately.
Brian - Windows 10 Home, 64-Bit, Version 1809, Build 17763
Explanations exist; they have existed for all time; there is always a well-known solution to every human problem — neat, plausible, and wrong.
~ H.L. Mencken, AKA The Sage of Baltimore