Synchronise two databases (but not really)
Yes, but by using tricks. MySQL has statement based replication, which causes the same SQL to be executed on the target as on the source. So the question is, can you write a SQL statement that does what you want? The answer is yes if you can exclude the rows in the target somehow. One way to do this would be a column that is 0 on all the sources, but which you update to 1 on the target by some mechanism (batch job, trigger, etc). Then when you execute the DELETE on the source WHERE ... AND is_target=0 the statement will do nothing when it is replicated. 

There is no way that I am aware of to simply have the target system filter out deletes (this can be done with with Shareplex so I assume GoldenGate too, but that costs money).