I have two panels and in this way I lock and unlock both panels and the panels will not allow any changes to their configuration. It does not lock each item separately, but if you apply this trick there is no need.
Otherwise in dconf editor (gui of gsettings) search “locked-down” and put the check.
org-> mate-> panel-> general-> locked down -> true
The two commads above correspond to this setting in dconf and the the effect is immediate
http://imgur.com/LH27N0Z
Tested on Ubuntu 14.04, 16.04 and Linux Mint Mate