How bad do you want to win? If all you want is martial efficacy that's super easy:
Always box a wrestler and always wrestle a boxer. In other words don't play their game. If you try to out grapple a grappler, without being one yourself, you're probably going to loose. As previously stated, most BJJ guys are generally not the greatest strikers, so having a good 1-2 cross and left hook in your tool box can be a life saver. Don't believe me? Try and hold you own with a decent amateur boxer. He/She will hold you at bay all day and work you over at will with nothing but a straight jab and good foot work. Bruce Lee emphasized this idea with the power lead.
Also, NEVER go to the ground with any one. BJJ practitioner, or not, the ground is a bad place to be, especially when he pulls that knife out of his pocket you didn't see, or his friends come out of hiding and start kicking out in the head while he holds you down. So don't even train for it, or you will do it in real life and it will backfire. As in nearly all tactical situations, the high ground (e.g. standing versus grappling) is best. For further proof consult the Isreali Self Defense and US military tactics, which strongly discourage going to ground at all costs.
As far as stymieing a BJJ practitioner in particular, you'd be amazed at how well simple tenkan motions and good ol' nikkyo can be. Try applying nikkyo the next time a BJJ grabs your lapel, then hold it for second or two, or three after they tap so they can fully appreciate its efficacy. Most of the BJJ guys have never felt a proper wrist lock and when they do, boy does it get their attention. It also makes them instantaneously less grabby. They're so often concerned with not getting choked out, that they forget about good ol' pain compliance (crude, but effective). It's also humbling from a psychological point of view because it is so quick and effective.
Of course, if none of that works and he does get you to the ground, get into full mount, take your thumbs and drive them into his eye sockets and start doing push ups (literally). I assure they WILL comply with great expedience. Another oldie but goodie is to physically bite their nose while in full mount. You can choose to bite it off fully at your discretion. Look at what a little bite on the ear did to "Iron" Mike Tyson. Image that was your nose and realize you're not 10% as tough as Mike. Heck, you can stop a Great White Shark if you hit him hard enough on the nose and do the eye gouge trick, which means a BJJ guy should is a piece of cake.
It's simply a matter of how far you're willing to go. I practice MARTIAL arts. Martial=military=killing/potentially lethal. If someone wants to study grapple-sport-competition-entertainment that's something else entirely and I could care less.
Good luck getting into full mount against a BJJ player, and if you somehow manage to get there, good luck staying there long enough to eye gouge or bite. By the way, the whole eye gouging defence is ridiculous anyway: check out this video http://www.youtube.com/watch?v=rFYDb9Ki5o0
. BJJ is the most martial (potentially lethal), martial art I have ever experienced; sport or not, the techniques are very real and deadly if needed.
The multiple opponent argument, never go to the ground; good idea. But what if you have to? Wouldn't it be nice to know how to quickly choke someone unconscious, or dislocate an arm, then get the hell out of there? Again, if you are mounted with multiple opponents the Bjj player knows many escapes. It is extremely short sighted thinking to just assume that you wont end up on the ground.
If you can afford it, and have the time, learn the ground game! Bjj + Aikido should be very complimentary. Just watch some Roy Dean clips for examples.