One of the best way to prevent unauthorized attempt to access SNMP is to use Access control list or ACLs. Use the inbound ACL on the interface and deny SNMP from the host that is accessing it without authorization.

In BGP, when a route map is applied to filter or manipulate prefixes, only the prefixes explicitly permitted by the route map are processed. If no additional permit statements exist to allow prefixes not matched in the first sequence, those prefixes are denied by default and will not be advertised.
In the provided configuration, the route map PRE-PEND permits only prefixes that match the PRE-PEND-PREFIX prefix list and applies AS-path prepending. Other prefixes that R1 might advertise are implicitly denied due to the absence of additional permit statements in the route map.
Add a permit statement (e.g., permit 20) without any set conditions in the route map. This ensures that prefixes not explicitly matched in earlier sequence numbers are still allowed to be advertised.