You used to be able to deselect the free trial of the Premium version but you now have to take it (in the first instance). I've found that the Premium trial can considerably slow down some of my machines, so I'd rather go nowhere near it at all.
However there's an alternative program, which used to be independent of Malwarebytes but has now been bought up by them, which is totally free (with no trial periods to annoy you) and, since it's now produced by Malwarebytes, should be every bit as good as its better-known big brother. So this is what I recommend nowadays:
https://www.malwarebytes.com/adwcleaner/