thats easy. Proxmox uses a rule based filter system, rules are processed according their priority.
Your case:
For example, if your spam filter rule has priority 80, just create a new rule with higher priority, for example 81. The rule should do the following.
- FROM: anybody
- TO: your LDAP group or LDAP accounts
- WHAT: anything
- WHEN: always
- ACTION: accept
"accept" is a final action and no rule after this one will trigger.