Is there any way to delete comments?

Not always, no. On your profile and your projects, you can easily just delete them with 1 click. On another users’ profiles and projects, you can ask them to delete the comment(s). If they’re anywhere else (like studios or forums), the only way to delete them is reporting them.

Basically what VioletCalico said. You can only delete them yourself if they're on your profile or your projects. Anywhere else, you have to ask for them to be deleted (for studios, report for deletion).

If it's on your profile, project, or studio, yes.
If it's on someone else's profile, project, or studio, no.
If it's on the forums and it's yours, l think you can report it to be deIeted.
If it's on the forums and it's someone else's, you could try but it's much more unlikely they'll delete it unless it's inappropriate.

Also, if the comment is not okay for Scratch, reporting it will also delete it if it's in your Scratch project, studio, or profile comments.