Absolutely.
Everyone who joins a clinical trial has the right to change their mind at any time and withdraw their participation. This means that you are free to withdraw from the study completely, or to refuse particular treatments or tests. Sometimes, however, this will make you ineligible to continue the study. If you are no longer eligible or no longer wish to continue the study, you will return to the care of the doctor who referred you to the research project.
You should not ever feel like you are required to participate in a clinical trial. If you feel that you have been pressured into participating in a clinical research project, you should contact the IRB listed in the informed consent form and tell them.