Depending on how badly you want to keep you pleco and how mush work you are willing to do, I just keep replanting mine until they take root finally and he can't knock them loose anymore. It is a little frustrating, but most plants get a good hold in 4-5 days and then we don't have any more problems until new plants are added. As a cautionary note, I tend to buy few few delicate plants, because I love my pleco and have no intention of removing him.
If you aren't in love with your pleco, trade the dude in on one of the suggestions above, some folks like fish tanks with plants, others like aquatic gardens with fish. to each their own.
dave