This seems to be a growing issue, so a new rule has been set in place in the rules channel. If anyone has encountered unwanted commission DMs and won't leave you alone, take screenshots, block them, then report it to staff. If this becomes too much of an issue, then non-friends will not be able to DM each other.
New rule:
7.) Commission DM.
As of now, DMs about commissions will be prohibited and will be kicked without warning. This will only happen if a member reports it to staff.