Most people deselect the option to take a trial of the 'PRO' version (and only use the free version), so the question doesn't arise for them anyway.
If you've opted for the trial and simply ignore the messages inviting you to pay for the PRO version it will automatically become the free version at the end of the trial period, so you'll still have Malwarebytes available on your computer. However, unlike the PRO version, the free version doesn't run 'in the background' (in the same way that anti-virus programs do); you have to remember to to run it 'on demand' from time to time. Other differences between the PRO and free versions are listed here:
https://www.malwarebytes.org/free/
Trial software deliberately leaves traces deep in your computer so that any attempt to keep on using it on a trial basis, time after time, will be detected and rejected, so the simple answer to your question is 'No'. You'll have to decide whether you want to pay for the full version or just continue with (what will become) the free version.