Is it ethical to refuse enrollment of a previously dishonest student if you are the sole instructor for a required course?
It's not up to you

It's reasonable for an instructor and syllabus to specify what happens in that course in certain circumstances. However, going beyond your particular course (including future semesters of it), any further academic punishments for plagiarism or cheating, including being restricted from taking or retaking certain courses, are up to the wider department, faculty, study program, or however your institution organizes things. There should be a universal (not made by and for a single course and instructor) policy on what are the wider consequences for certain acts of plagiarism. Perhaps that policy prescribes that being forbidden to retake that course (or some time limit) is an appropriate punishment, in which case your actions are valid. But if it does not prescribe that students will be prevented from taking future courses, then it's not up to the course syllabus or the instructor to unilaterally decide whether the student is eligible or not.

Note that I'm not saying that you should waive the policy just because it disrupts the path to graduation - there certainly can be academic dishonesty policies that may prevent students from graduating in case of plagiarism. But the question does not mention any specific wider policies, and if it eventually does come to a ad-hoc decision regarding a particular student, that still would not be your decision to make, this should be escalated to your superiors.