In one of your template files there should be a <title></title> HTML element that has the Wordpress function inside of it which generates the title of the document. I don’t know how your template hierarchy is set up, but the first place for you too look is in your template’s header.php file if there is one and look for something like this:
The first argument in the wp_title function call is the separator between the post title and the name of your website. Mine is a pipe character (|). Between the two single quotes you can put in one or more spaces that will place spaces between your post title and site name. Something like this:
Make sure you save the file. And it would be a good idea to make a backup copy of your files before you modify them.
If changing header.php does not work, you are going to have to explore a few more of your template files to determine where in the Wordpress template hierarchy your <title> text is coming from. Try looking in index.php, category.php, and then work your way through the rest of the files. It has to be in there somewhere. Now that you know what you are looking for, you are going to have to search through your template files and find it.