If all the evidence you have collected is a flood of packets with source IP addresses from one particular AS, you have likely jumped to the wrong conclusion. A more likely explanation would be that those source IPs are spoofed.
A reflection/amplification attack involves sending lots of packets spoofing the source IP address of a victim. If this is actually what is happening, and you have servers in your network, that can amplify an attack, then the network you are accusing of an attack is actually the victim, and you are aiding the attacker.
In such a situation the solution is not to apply any sort of traffic engineering, but rather to configure your servers such that they cannot be used in an amplification attack. How to do this is not really a network engineering question.
It is of course possible, that all the packets are originating from one AS. With co-operation from the offending AS, you could get confirmation, that the packets do in fact originate from their AS. However with that level of co-operation, you could also get the attack blocked at the source.
If we assume you have through some method I haven't thought about gotten confirmation the packets are really originating from the AS you think, and that you cannot get it blocked at the source and instead want to block it through means of BGP, then I have read about a somewhat risky method to achieve this. The idea is that you prepend an AS path to the route you are announcing. In this prepended AS path you include the AS number of the source of those packets.
When the announcement reaches the BGP routers in the offending AS, they are going to detect a loop and drop the announcement. Meanwhile the rest of the world won't see a loop and accept the announcement.
That's the theory. Whether it will actually work in practice depends on a few different factors. For example it depends on actually using the AS number the packets are originating from, which could be different from the AS number announcing those IP addresses. (Such difference could be legitimate or due to spoofing.)
It also depends on your upstream not filtering the route if they find the AS path suspicious. Moreover networks further away from you may also drop your route for example if they also has had bad experiences with the offending AS and have decided to drop all routes from there.
It's your call whether this approach is worth the risk.
(I would have linked to the source for this approach, if I could find it again.)