how can i block mikrotik discovery protocol at pppoe mikrotik router?

Just disable the interface associated with the pppoe access concentrator in the menu:

IP ---- > neighbors ----> Discovery Interfaces.

Regards.

clients can see eachother when they login their cpe

"see eachother" where? In the neighbor menu? Please clarify the exact problem with examples

pppoe server -------------- Client1 (connected via fast ethernet) neig. enabled
(neig. disabled ) Client2 (connected via fast ethernet) neig. enabled


when client1 and 2 can see them at neig. menu at their cpes

i want to block all discovery protocol between cpes and pppoe server

??

Please Disable de option forward in wireless menu in your BS, only active the authenticate option.