Thank pozitron7 for the timely help!(Source)
He provided me three ways.
I preferred the third way, 'cause I don't need to reinstall Isso or reuse the old blog to delete the ones I want to delete.
Here's the way:
Edit directly the database file, which is by default
/var/lib/isso/comments.db(If ur file is not in that path, replace it with ur own, the same below).
Copy this file (create 2 copies in case you'll mess something up and won't notice it immediately) and edit it using Sqlite3:
cp /var/lib/isso/comments.db ~/ cp /var/lib/isso/comments.db ~/original_comments.db cd sqlite3
Have an sqlite> prompt then.
Use sqlite3 syntax to find and delete comment, e.g.:
sqlite> ATTACH 'comments.db' as isso; sqlite> SELECT id,author,text from isso.comments;
Imagine that id of some comment you want to delete is 852. To delete it you should do:
sqlite> DELETE FROM isso.comments WHERE id=852;
This solved my problem, not perfectly, when the ID of all the comments stayed the same, which meant the deleted comments still occupied their original ID.
So I asked if there was a way to solve this. Then he offered the remedy which tackled my barrier again:
To change id of a comment from 892 to 890 in Sqlite3 you can do:
sqlite> ATTACH 'comments.db' as isso; sqlite> UPDATE isso.comments SET id='890' WHERE id=892;