When you say you have imported a GEDCOM do you mean you have recently File Merged a GEDCOM into your main Project?
I can understand why that might cause a problem if many of the merged records are of no interest to you.
If it is only a recent merge, can you revert your main Project to a Backup taken just before the merge?
Then the way forward would be to create a separate Project from that GEDCOM and split out the people of no interest.
People who are not related to anyone in your tree will have a different Relationship Pool number, so are easily identified.
We have discussed Relationship Pool numbers here before but do you need a reminder?
The first step is to run the Search For Orphans standard Query which despite its name actually just lists people by Pool number.
If you cannot revert your main Project then that Query would also help you find unrelated people.