Thanks
Basically the rules are you can't send unsolicited PMs. You can't send SPAM.
Besides that there are some limits on the number of PMs you can send.
It's related with "Limits on posting":
https://bitcointalksearch.org/topic/forum-rankspositionsbadges-what-do-those-shiny-coins-under-my-name-mean-178608
You also forgot extortion, (Blackmailing people into giving you money.)
Also, to OP, sometimes threads are locked for spam, I.e. excessive bumping. Or, it could be as someone else mentioned, your actions may have lead to your thread being locked.