I’m not sure what the sign means regarding trade parking, is there any bylaw which has authorised the sign?, If not it really has no effect on the legaility of anyone parking there. In any case, bylaws cannot be enforced against anyone other than residents of the complex.
I sympathise that it is not always possible to inform EC when any tradesperson may need to attend your unit, you may have for example an urgent plumbing or electrical problem which needs attention, and the tradesperson needs to park in a convenient location for access to work tools etc.
It should be perfectly obvious that the person there had a legitimate reason for parking in that spot, but some overzealous committee member has taken it upon themselves to throw their weight around. If they have caused damage to the vehicle by stickering it then the tradesperson concerned could rightfully seek compensation for the damage caused.
I think it would be quite appropriate for you to raise the foregoing matters with the EC and advise them that the signage needs to be clarified and that they may be held liable for damage caused to a vehicle by stickering it, and you as a member of the OC will not accept liability for any inappropriate conduct.