In this topology, R3 and R7 are in fact the ABRs. If we apply an area filtering inbound direction (with the "area area-number filter-list prefix ... in" commands) on R3 and R7 then the ABRs will filter prefixes that enter this area. For example, on R7 we use the command "area 2 filter-list prefix Deny_Lo_R6 in" command then the Lo interface of R6 will not be advertised into area 2.
Note: The "area area-number filter-list prefix ... out" (out direction) will prevent prefixes from advertising outside of this area although the ABR still learns about these prefixes.
In particular, the following commands will prevent R6 loopback from getting into Area 3 on R3 (of course we have to configure the same thing on R7):
R3(config)#ip prefix-list PREVENT_R1_LO deny 1.1.1.1/32
R3(config)#ip prefix-list PREVENT_R1_LO permit 0.0.0.0/0 le 32
R3(config)#router ospf 1
R3(config-router)#area 3 filter-list prefix PREVENT_R1_LO in
Notice that we applied the filter-list for area 3, not area 0.
Just for your information, if we want to use an outbound filter-list to prevent R6 loopback from getting into Area 2 & 3, we can do it on R3 & R7 for area 1 with the following commands:
R7(config)#ip prefix-list PREVENT_R6_LO deny 1.1.1.1/32
R7(config)#ip prefix-list PREVENT_R6_LO permit 0.0.0.0/0 le 32
R7(config)#router ospf 1
R7(config-router)#area 1 filter-list prefix PREVENT_R6_LO out
But with these commands, R6 loopback is also not advertised into area 0, which is not satisfied the requirement "prevent the R6 loopback from getting into Area 2 and Area 3 from Area 0".
We found a good tutorial with same topology and requirement at https://ccieblog.co.uk/ospf/ospf- filter-lists