First, no one owns the phrase “lives matters.” Anyone can use it to construct or transmit any message.
Second, with language, it can’t be thievery as it’s a social construction that is under constant change. If you say that someone can steal a part of a language (a word or structure), then all languages and people are thieves as many words that we use in our daily lives came from the transformation or using old parts of ancients languages or foreign ones.
Third, you can hate the utilization of the phrase with other meanings, but, in the end, it’s part the right of free speech to be able to express ourselves in the way we wanted.
Fourth, If you still defend your idea I could have the right to be angry at the fact that the LGTBQ community stolen the symbol of the Incas civilization (the rainbow flag) and strip it of the meaning of unity of the four Suyos that integrated the culture. How dare they to steal a symbol of my culture and not only change their meaning without permission but make it theirs. I should demand reparations and for them to stop using that symbol that doesn’t belong to them. Even the first person who stole the flag from us in the first place was a white gay man. FUF
Fifth, maybe be more direct and say “I don’t want anyone, but black people using the phrase “lives matter” and even then they have to use it under the only authorized combination of #blacklivesmatter.”