1. Long-press the number in the phone log. Choose Block.
2. Telemarketers use fake caller IDs, so the same telemarketer can call you over and over from different "numbers" while still using the same phone. (If it's your area code and an exchange close to you, you're more likely to answer.) You can't block the telemarketer, just the fake number he's using for this call. (And if it's a robocaller, they alternate numbers for each call.)