LRM spamming is usually something like this: build the first Capital Ship, scouts, a minimal number of Light Frigates, then tech to LRMs (T1 for Vasari, T2 for TEC) and building them exclusively. LRMs are probably the best unit in the game for their cost and population and are so low on the tech tree that in many games they're almost all that's needed. Generally the only way to fight this are Strikecraft, Flak, Heavy Cruisers, or spam your own LRMs, all of which have problems.
Strikecraft is an option, since they can do considerable damage to LRMs, but they are easily destroyed by even small numbers of flak, it's no longer pure LRM spam then, but is enough to nullify that strategy.
Flak is another, since flak of your own takes a long time for LRMs to kill and can do decent damage to LRMs, although it takes a while. The problem with it is that LRMs are good against the majority of units in the game, while flak are only really good for a few roles. So unless your opponent jumps from LRMs to Carriers, your flak will be obsolete when they change their tune (and if most of the LRMs just flee, they can come back later and still be just as effective).
And then there's Heavy Cruisers. They do outstanding damage to LRMs, can usually close with fairly little damage, and once they get in close they chew through LRMs like candy. Trouble is they're T5 and not easy to tech to before there are tons of LRMs around, and with their long build times it's tough to amass them fast enough to brawl with the LRMs in time. Heavy Cruisers are good, but trickling them one by one into an LRM blob is not a good idea.
You can always spam your own LRMs, and if you have a better economy can win (especially if you mix flak in and your opponent doesn't). But saying to best counter a strategy is to do it yourself is kinda weak.
Very cost effective, versatile, spammable unit.
