Can I return my block of IP address space if it gets filtered?

No. RIRs such as the RIPE NCC are responsible for the allocation and assignment of IP address space, but not for the routability of address space.

You should contact the networks that are blocking your IP addresses and request that they remove the filter affecting your network traffic. If you have problems identifying the networks that are blocking you, or you don't receive a response from them, the RIPE NCC will assist you and try to establish a direct contact between the impacted parties. For help with this, send an email to lir-help _at_ ripe _dot_ net.