If your cellphone provider doesn't have an option to block numbers, you can try this:
Set up a contact on your flip phone with a name of .SCAM or some such. Note the DOT in front of SCAM. That should keep the contact info at the top of your contacts list in most cases, easy to find.
Anytime a call comes in that you want to block, save the number to that contact. You can usually add multiple numbers to a single contact, so do that if there are other number to block. It doesn't matter which type of phone number you select in the contact listing: home, cell, work, other, etc., those are not relevant here.
Then assign a silent ring tone to the .SCAM contact entry, or select "SEND DIRECTLY TO VOICEMAIL" if that option exists.
If you are limited to the number of phone numbers you can store to the entry for the contact named .SCAM, then go with .SCAM2, .SCAM3, etc. Hoping it doesn't get that bad, though.
If you can't assign a silent ring tone, at least you will see .SCAM when the call comes in and you'll know to reject it manually. Good luck!