Does your carrier’s online account management allow you to block numbers yourself? That will prevent them from leaving voicemails. If they don’t let you do it yourself online, you’ll probably have to call, and regardless, there might be a charge per number - the last time I looked into it, it was $10 to block a number for my Verizon account.
You can reset your voicemail, which on some carriers will result in callers getting the, “This voicemail box has not been set up yet,” outgoing message (which will not let them leave a message). From the quick search I did, it looks like it’s a pretty carrier-specific process, though, so you’ll need to search “<carrier name> reset voicemail” to get anywhere.