If you are experiencing newly imported contacts getting automatically enrolled into a sequence, it is very likely the case that the new contacts are matching the filters you have for a particular Smartlist that you also have set to auto-enroll prospects into a sequence.
In these instances, we would suggest taking a look at the SmartList you have set for auto-enrollment, under the sequence you are noticing this occurring, and matching the filter parameters tied to that SmartList from your Contacts page.
Typically, an adjustment to your SmartList filter logic will help with this, but if you are using Sequence filters on SmartLists to enroll into other Sequences, you may need to dig a bit deeper into the Auto-Enrollment SmartLists for the Sequence you have as a filter condition.
If you are still facing issues after investigating the above points, we would kindly request a thorough example and cases where you are importing leads and they are getting automatically enrolled in your sequence, and then proceed to open up an issue with our support team sharing these specifics.