I am not aware of a way to remove a partner. You can change the type of partner, but that's not what you need.
I believe you could download your GEDCOM file, edit it directly, and then upload it again. You would be looking for a group of lines that compromise a family record. It would probably look something like this:
1 FAMS @F1@
0 @F1@ FAM
1 HUSB @I125@
1 WIFE @I245@
1 _CURRENT Y
1 _PRIMARY Y
Search for the names of the people you inadvertently married, and note that one of the the enties above their name will start with SUBM. The characters after that is the identifier for that person. Now find the other person and note their identifier.
Now you're looking for a family record by searching for lines like the HUSB and WIFE lines in the example above. When you find them, delete the lines associated with that family. Save the GEDCOM file and you can create a new family tree from your account page and upload that corrected GEDCOM file. The upload button is in the lower left after you make a create a new family tree.
Don't delete your old family tree until everything looks okay.
Or you can just delete one of the married peopel and recreate them.