No worries, the "thanks" button will not lock your thread. It is only visible and available to the person who starts the thread. And, as far as I know, it is no longer functional. If this is true, I'm not sure why the icon/button still appears.
The Forum originally had a feature which allowed members to "vote" on other member's comments with a plus or minus vote, which contributed to the members "reputation", expressed as a cumulative number of "points". "Thanks" given by the starter of a topic to replies, also contributed to the recipients "reputation".
By consensus, we collectively decided to disable this function of the Forum. I guess it's possible that the "thanks" function was not disabled, although previous thanks votes no longer appear.