You can avoid such behaviour in many different ways.
You can use the Paste as text option
Instead of using the Visual editor switch to the Text editor.
A third alternative would be to use the Clear formatting option after pasting your text.
Found that the issue is TinyMCE. Here is a solution that prevents the spans from being added on paste: https://bionicteaching.com/clean-google-doc-cut-paste-into-wordpress-editor/
Add this to functions.php:
//fix cut paste drama from https://jonathannicol.com/blog/2015/02/19/clean-pasted-text-in-wordpress/