No clinical trial can exclude a person from participating without a research-related rationale. Each study has particular patient characteristics it is looking for.
All clinical research is conducted according to plan. The plan outlines who may participate based on particular characteristics including age, medical history, diagnoses. These are called inclusion/exclusion criteria. These criteria or characteristics qualify a person for a particular study are the inclusion criteria. The characteristics that disqualify a person from a study are the exclusion criteria. These criteria are in place so that patients are kept safe and the research is designed to study the appropriate population.