If there is some work around please post
A work-around exists through changing the Number formatting of the Reference line. Right click the reference line, click format, and on the Numbers option, select Custom. In this box, you can type whatever text you want to precede the number in quote, then follow the standard formatting syntax to specify the desired number format outside of the quotation marks.
There are some limitations however (if there is some calculation on the number, you cannot remove the explanation of that calculation: e,g, "Average of..", "Sum of ..." ). This works best if you're using a constant value, because only the value automatically appears by default, and you can add any extra text in the custom format.
See more details in this thread: Customizing (or removing) tooltips on reference lines
Hope this helps,