If dumpers can dump coins and tokens in a reasonable way it won't affect the value much , lose in value when dumping happens if a dumper dumps huge amount of the token at once , this is a wrong way to dump coins, this is why till date some devs put locks on their tokens ,some even release percent by percent on term basis just to avoid huge dump.
The best way to dump is not to dump in an instant or a whole at once
Nothing can stop them. With tokens listed on many exchanges, dumpers will deposit their money to multiple exchanges to sell. You cannot stop them. With tokens listed on many exchanges, dumpers will deposit their money to multiple exchanges to sell at the best price. The only way now is to distribute the token according to a vesting schedule.
This call of OP is a call that can never happen, for as long as we have people without conscience, it is only conscience that can prick the dumpers and push them to make a reasonable move as regards their dumping of token, but since money is involve, the God of mammon would not allow them to think straight.
So, the best way to just handle this is for developers to continue working hard to build the project, if they have a real product with use case, no matter the dumping that has occurred, there will always be opportunities to get investors or users that will continuously place demand on the remaining token which will turn the table round and make the value starts to grow again, then they also need to reduce the amount of token they give as interest.