try this solution to block a particular mobile number, if the contact is saved in your phonebook, do these.
1. press menu,
2. select settings
3. select application settings
4. select call
5. select all calls
6. select auto reject
7. scroll down and select reject list and add number.
8. select search and it will give an option
9. select from two options, call log and phonebook
10 select phonebook (this is because the numbers were saved in your phonebook)
11, once set and configured, check and enable auto activation
12. click save and exit the settings.
if the contacts is not on your phonebook.
follow the steps 1-9 but step 10 will be your call log not the phone book.
10. select call log
11. it will give you the phone numbers of all the person who contacted you, including those who sent you a message,
12. at the right upper corner you will see all, filter it through received message and it will give you those who sent you messages.
13. select from the contacts or numbers from the list.
14, select and enable auto activation
15, select save and exit the menu