I've used MBAM for several years on multiple computers, and never been asked to register or pay. I guess you must have installed a trial version of the full programme?
Uninstalling this from the control panel probably won't help, as there are often configuration files/folders left behind which will remember the old version.
Try "Revo Uninstaller" which, if you run it in the Advanced mode, will search for all the remnants, including registry entries. Be VERY careful to ensure that only the bolded ones are ticked before removing them! This should completely remove whatever version you currently have. Then making a fresh install of the version linked by BOAC should stop any prompts to register or pay. I have noticed that the latest version does try and encourage you to update, but doesn't force you - just be careful where you click!
There's a "portable" version of Revo, which doesn't need installing, and can be run from a pendrive or a folder anywhere on your PC:
http://www.revouninstaller.com/downl...ninstaller.zip Just unzip this download and click the .exe file inside. I've used this extensively and found it works well.