Ah! The HOB one, awesome. I was going to suggest replacing the filter if it was the internal one.....those things have a really strong flow, and it is really difficult to baffle them. xP I had one in a 10 gallon a while back and the flow even troubled the non-betta fish I had in there, I REALLY disliked it.
I have a similar(older I think)Tetra Whisper HOB filter model in my 5 gallon that has lasted me over three years so far, I really like it.
You should be able to secure a bit of filter sponge right over the outflow with a rubber band or hair band that doesn't have any metal. Like so:
Just turn the filter off, put the sponge over the outflow, slip the rubber/hair band up and over the sponge and around the back of where the outflow-area of the filter hangs into the tank and it should stay in place just fine. You may need to shift the intake a little to slip it all into place(you should be able to wiggle it in behind where the intake goes up into the filter), but it shouldn't be a problem. The band will stretch out over time, so every once in a while you'll have to replace it, but other then that this method I've found works very well.