[BirdOPrey5]

You could edit your showthread template and find the code that displays the "Post Reply" button, then surround that code in a template conditional:

<if condition="$show['member']">
{original code here}
</if>

That will show the code (the post reply button) only to people who are members (not guests).

If you need to get fancy and do this only in certain forums then look up the article on conditionals in the vb3 articles section here and you will see more conditions you can use to limit the code per forum.