I already opened a case to Solarwinds and they said it is routers (SRX650 & SRX2XX series) fault 
. So here is the solution for you. This query will clear the "Not Available" neighbours. If you want you can schedule it like I do.
delete from NPM_RoutingNeighbor WHERE IsDeleted = 1 AND LastChange < DateAdd(day, -1, getdate())