"Stop talking about romantic things" literally means stop talking about scenery, flowers, the moon and other topics. It is usually suggested not to talk about romantic things, that is, not to talk about topics related to men's and women's feelings, suggesting not to involve topics in men's and women's affairs. It is usually used to restrict or prevent people from speaking.
And "only talking about romance" means only talking about scenery, flowers, the moon and the like, and not involving other topics. This phrase does not restrict or limit the meaning of conversation, but simply refers to talking about some romantic topics, such as scenery, flowers and the moon.
Therefore, "don't talk about romance" and "just talk about romance" are slightly different in tone and meaning.