After disabling extension in Magento it will have no impact to the store just like it would be not installed at all. Some extensions consist of just one module other of few. You may disable whole extension or just some part of it.

Fastest and easiest way to temporary/permanently disable extension or some of it's module(s) in Magento is simply removing module activation .xml file(s) from app/etc/modules and refreshing Magento cache (all cache types in system->cache management or simply pushing "Refresh cache" button if our "Refresh cache " extension used) after it.

If during extension installation some error happened and you are not able to access admin panel you can clear cache manually by renaming or deleting var/cache folder.

Disabling MANAdev extensions

MANAdev extensions are very flexible and usually consist of several modules. MANAdev extension module activation file names start with Mana  like Mana_Filters.xml or ManaPro_FilterCheckboxes.xml. To disable MANAdev extension or some part of it remove corresponding Mana*.xml files from app/etc/modules and refresh all Magento cache types after it.

Other way is to open module activation .xml file with text editor (you will notice <active>true</active> which means module is enabled and running). Disable extension by changing it to <active>false</active> and refresh cache as described above. This method is good if you need to disable one module.

At last, you can disable module output in System->Configuration->Advanced section. In this case module is still up and running, but its blocks and templates eliminated from frontend and backend pages and visually module seems to disappear. Modules with disabled output can still react to different events happening inside Magento and amend/change standard functionality.